September 25, 2018, was a stress-filled day at Instagram. The prior evening, co-founders Kevin Systrom and Mike Krieger had announced their sudden departures from the eight-year-old company, leaving employees stunned and uncertain about the future of the photo-sharing app.

They didn't know who to ask about current projects since Systrom and Krieger had been running the show even after selling the business to Facebook for $1 billion in 2012. So employees took to Workplace, Facebook's internal social network, to air their frustrations.

One former employee who left the company early this year described the general mood around the office to CNBC as "disheartening."

The team would get some clarity a week later. That's when Adam Mosseri, a 10-year veteran of Facebook, was elevated to head of Instagram. In a blog post announcing the move, Instagram included a picture of a smiling Mosseri sitting on a couch with Systrom and Krieger grinning widely on either side.

Mosseri, a product designer who'd climbed the ranks of Facebook, was viewed by Instagram employees as an acceptable choice, if not exactly celebrated, according to former and current staffers. He'd joined Instagram just five months earlier after previously managing News Feed, the central place Facebook users go to check up on their friends.

At age 36, Mosseri now has one of the most challenging and consequential jobs at Facebook, running a unit with more than a billion monthly users that's been valued by analysts at upwards of $100 billion — representing about one-fifth of Facebook's market cap. Instagram is also the most popular social network among teens, an age group that's shown declining interest in the main Facebook service, and it benefits from the fact that most Americans don't know the site is owned by Facebook, according to a study last year.

As CEO Mark Zuckerberg turns to Instagram for growth, Mosseri has to appease an employee base that wants to keep its independence from Facebook, whose reputation has taken a beating since the 2016 presidential election. Facebook is busy rolling out more tools for advertisers to pull together their campaigns across sites and is working to integrate the infrastructure of Instagram with Facebook Messenger and WhatsApp so they're all similarly encrypted.

"Facebook has great expectations from Instagram as a source of revenue," said Eric Meyerson, a former Facebook director who worked with Mosseri on a few projects before leaving the company in 2017. "Figuring out how to better monetize Instagram without ruining the community or negatively affecting the user experience is just going to be the heaviest business challenge."

Mosseri has the added task of succeeding two co-founders and leaders who were beloved by employees, in part for maintaining a high level of autonomy. Systrom was known as a brilliant project visionary, while Krieger was the heart and soul of the service, coding features like the popular neon pen font in Instagram Stories, the user-generated photos and videos that take over an entire screen and disappear after a day.

Looming over everything is a growing chorus of calls to break off Instagram amid concerns that Facebook has too much power. Senator Elizabeth Warren of Massachusetts, a Democratic presidential candidate, has said the Instagram acquisition should be undone, a sentiment echoed this month by Facebook co-founder Chris Hughes, who wrote in a New York Times op-ed that Facebook, Instagram and WhatsApp should become "distinct companies."

CNBC spoke with 20 current and former Facebook and Instagram employees for this story who have worked with Mosseri. Most of them requested anonymity because of confidentiality agreements.

An Instagram spokesperson declined to comment. Mosseri was not made available for an interview.

Housed in buildings 24 and 25 on Facebook's main campus in Menlo Park, California, Instagram now employs about 1,000 people within Facebook's 38,000-person workforce. Since Facebook opened an office in a downtown San Francisco skyscraper last year, more Instagram employees have moved north to the city.

That group includes Mosseri, who sometimes rides to the office on the BART train and works at an open desk on a floor surrounded by Instagram leadership. He still splits his time between the offices, alternating for his weekly all-hands meetings, according to employees.

While Mosseri has fully embedded himself in Instagram, there's plenty of internal anxiety about having a boss from Zuckerberg's inner circle. One employee described a general feeling of "discomfort," while a former employee who left after Mosseri took over, said that "resentment against Zuckerberg leads to resentment against Mosseri."

Within Facebook, Instagram has long been viewed as the coolest part of the company. Before starting, employees go through a weeks-long bootcamp. At the end, many of the new recruits have the power to choose which group to join, assuming that team wants them. Incoming employees routinely pick Instagram as their top choice.

But lately, Instagram has been suffering some brain drain, and not just the departure of the founders. Kevin Weil, the former vice president of Product, James Everingham, the head of Engineering, and Hui Ding, director of infrastructure engineering, all moved over to the company's new blockchain group, which is keeping quiet about its work but is reportedly developing its own cryptocurrency. Led by David Marcus, PayPal's former president, the group has lured a number of other Instagram employees because it's now seen by many internally as the most anti-Facebook part of the company.

Mosseri nabbed several Facebook executives to fill senior Instagram roles, including putting Nam Nguyen in charge of engineering and naming Luke Woods as vice president and head of design. He's still looking for a chief operating officer, according to a report earlier this month from Cheddar.

At Instagram, much of the focus is on fixing problems that resulted from out-of-control growth. Two of Mosseri's biggest priorities, employees say, are the company's anti-bullying efforts and Facebook's plan to pull together Instagram with WhatsApp and Messenger.

Tamping down bullying is the responsibility of the well-being team, which has its own website and says it's committed "to building a safe, kind and supportive platform." The issue has gained publicity following a report last year from Pew Research that 59% of U.S. teens have faced online bullying or harassment. A story the following month in The Atlantic singled out Instagram for providing "a uniquely powerful set of tools" to enable bullying.

Mosseri and his deputies are using their public appearances to discuss the actions Instagram is taking. At Facebook's annual F8 developer conference in April, Mosseri said the company was experimenting with hiding "like" counts so hanging out on Instagram doesn't "feel like a competition."

"What we aspire to do — and this will take years, I want to be clear — is to lead the fight against online bullying," said Mosseri, who often addresses the issue in Instagram's all-hands meetings and in his weekly company-wide email.

It's not just bullying that he's attacking. Two months earlier, Mosseri put out a blog post announcing four changes to promote safety for the "most vulnerable people" on the platform, including banning images that show self-harm and providing resources to people who are posting and searching for that sort of content.

Then there's Zuckerberg's new emphasis on privacy, which he declared earlier this year is the future direction of the company. As part of that shift, Zuckerberg plans to link the back end of messaging services — Messenger, Instagram and WhatsApp — so they're all jointly encrypted. Some Instagram employees say they're concerned about Mosseri's willingness to listen to their feedback in the process and communicate it to Zuckerberg, rather than just following the lead of headquarters.

Zuckerberg and Mosseri have been in lockstep agreement up to this point on the most public matter involving Instagram — that it should be pulled apart from Facebook. Zuckerberg said such a radical move wouldn't solve Facebook's problems and would actually make them more difficult to fix. Mosseri tweeted the same idea on May 13, a few days after Hughes's op-ed.

"Breaking up Facebook would make it exponentially harder for us at Instagram to address problems on our platform," Mosseri wrote. "For most safety issues, from hate speech and violence to misinformation, we rely heavily on the teams and technology built out by the Facebook teams. Sharing data is also incredibly important — if we identify a bad actor on one platform we can remove them from the others."

They may be right, but Zuckerberg also knows Instagram is vital as a business, considering the flattening growth of Facebook's user base. Instagram has been at the forefront of Facebook's push into Stories, which Zuckerberg has said will eventually deliver more revenue than the Facebook News Feed. Instagram counts more than 500 million daily users for its Stories feature.

A bulky New Yorker and graduate of New York University who can often be seen wearing light transparent glasses, Mosseri was influenced by Facebook even before his first day at the company in 2008.

In 2007, he was hired as one of the first employees at TokBox, a San Francisco early-stage startup that was building a video chat service. He was known around the office for his sense of humor, often communicating with colleagues in a Chewbacca voice, an impersonation he carried with him to Facebook, former co-workers say. Above all, Mosseri had a reputation for clean design, which relied heavily on a color palette of blue, light blue and gray.

"A lot of us joked, 'Hey dude, why don't you just apply for a job at Facebook? You'll fit right in,'" said Bartosz Solowiej, who worked with Mosseri at TokBox.

His TokBox teammates weren't surprised when Mosseri told them not long after that Facebook had hired him as a product designer. At Facebook, Mosseri made an immediate splash, shipping his first lines of code on his first day, according to two former Facebook co-workers. His consideration for the placement of advertisements within his designs gave him credibility among the business folks.

He joined an intramural soccer squad at Facebook and flashed his striker skills on the field. He got the only extra-large jersey, which had the number 00 on the back — his teammates nicknamed it "Bagels." On and off the pitch, colleagues said nice things about him. A former member of Facebook's security staff said Mosseri and his wife, Monica, who he met at Facebook, treated everyone at the company with respect, regardless of where they were in the hierarchy.

Mosseri has had to deal with bumps on his way to Facebook's upper ranks.

He was the product manager of Facebook Home, the company's attempt at a smartphone user interface with a design that included face bubbles popping up on the screen as users chatted with their friends. The product was an immediate flop and was widely mocked.

By the time of the Home's troubled launch in 2013, Mosseri had moved on to managing mobile product managers. Along with Tom Alison, a vice president of engineering, he established what would become the News Feed team, running the most important part of the core Facebook app.

Mosseri was promoted to vice president in 2016, and he shifted to Instagram in May 2018, amid a reorganization of Zuckerberg's leadership team, before ultimately landing the top job.

Krishna Gade, who worked for Mosseri as a News Feed engineering manager, said the struggle to scale Instagram as a unique property inside Facebook is real.

"It's easy to get lost and make Instagram look more like Facebook, and that would be detrimental," Gade said. "Instagram has a unique experience."

Gade, who's now founder and CEO of a start-up called Fiddler Labs, has seen Mosseri face adversity. He said his boss was key to establishing the Integrity team, which works to eliminate fake news. That became an important initiative in the aftermath of the 2016 election, as concerns rose about Facebook's role in allowing the spread of misinformation and enabling foreign interference in U.S. democracy.

In late 2017 and early 2018, Mosseri was involved in a series of meetings about how Facebook would deal with the issue going forward, according to people with knowledge of the matter. In those sessions, Mosseri supported aggressively removing fake news across Facebook's services, the people said.

On multiple occasions, they said, Mosseri squared off with Joel Kaplan, Facebook's vice president of U.S. public policy and a former member of the George W. Bush Administration who stirred controversy last year for showing up at the confirmation hearing for Supreme Court Justice Brett Kavanaugh.

Mosseri, who's donated to Democratic political campaigns in past years, advocated for completely removing fake news from the company's News Feed, rather than simply pushing it down the rankings, the people said. He also lobbied for the removal of far right outlet Breitbart News from the list of publications that receive preferential treatment on the company's News Feed, and opposed partnering with the Daily Caller, founded by Fox News's Tucker Carlson, for fact-checking.

Kaplan argued that Facebook couldn't afford to appear biased against conservative media, but Mosseri countered by focusing on the difference between showing bias and banning objectively false information. On one occasion, the debate got so heated that Elliot Schrage, Facebook's former vice president of communications and public policy, had to calm Mosseri down, sources said.

Mosseri was ultimately unsuccessful. Breitbart still gets equal treatment with other major publications, and last month Facebook added Check Your Fact, the Daily Caller's fact-checking site, as a partner.

But in the process, Mosseri showed a level of feistiness that he'll need for the many battles ahead inside Instagram and Facebook and on the outside, where big tech skeptics are getting louder by the day.

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