Mosseri, who has been at the helm of Instagram since 2018, is among the more than two dozen witnesses that the Federal Trade Commission has called to testify in the case.
The FTC argues in its case against Meta that the company violated antitrust laws when it "helped cement" an illegal monopoly in the social networking market with its acquisitions of Instagram in 2012 and the messaging app WhatsApp two years later.
The case, to be decided by Judge James Boasberg, could be one of the most consequential antitrust trials in years. If FTC regulators have their way, Meta could be forced to sell off WhatsApp and Instagram.
Mosseri began his tenure at Meta, formerly called Facebook, in 2008. Here are five insights and revelations we learned from his more than six hours of testimony:
Mark Zuckerberg's 'strained' relationship with Instagram's founders
Mosseri recalled a 2018 email he sent to Meta CEO and founder Mark Zuckerberg while on paternity leave, warning that Instagram cofounders Kevin Systrom and Mike Krieger were increasingly frustrated with strategy changes.
He wrote that it was "hard for me to get a read on what's going on as the relationship was strained."
Mosseri cited two core tensions: Zuckerberg's belief that slowing Instagram's growth would benefit Meta overall.
Mosseri acted as a mediator, relaying concerns between the founders and Facebook leadership.
These tensions foreshadowed a deeper rift that culminated in the cofounders' departure later that year, a turning point that saw Mosseri take over the reins at Instagram.
Worry over TikTok cutting into Instagram's growth
TikTok's meteoric rise was a massive threat to Instagram, Meta has argued.
"TikTok is probably the fiercest competition we have faced during my tenure at the company," Mosseri testified on Thursday.
According to internal Meta documents presented in court, TikTok was a "big concern" in 2019, just as the ByteDance-owned app was taking off. Instagram data scientists presented a "conservative estimate" that 40% of Instagram's year-over-year decline in time spent was due to TikTok. Specifically in the US, Instagram estimated a 23% decline in time spent.
Instagram would go on to launch its own short-form video product, Reels, in 2020.
Mosseri also testified that he briefed Zuckerberg "very often" about the competition with TikTok, adding that there was a monthly dinner with the most senior executives at Meta where this would come up.
"It became kind of a hazing ritual for me to give an update on Reels," Mosseri said.
Mosseri's 'biggest mistake'
On the stand, Mosseri testified that Instagram's first version of Reels was his "biggest mistake," built on the "not a sound foundation" of Stories, which the feature was initially built into.
The feature flopped and was ultimately scrapped after nearly a year. Mosseri said before he joined Instagram that it tried another venture to compete with TikTok called IGTV — that too failed.
Instagram pivoted by relaunching Reels as a dedicated feature in the main feed, a reboot that finally gained traction amid the pandemic and TikTok's rapid rise. Mosseri said that the company "could have and should have been more aggressive" in responding to what he called Meta's fiercest competitor.
Hundreds of millions on content creators
Instagram's fight with TikTok and other apps is just beginning, Mosseri testified. Mosseri said that one of the biggest fights right now is over future creators, those who are just beginning to make content or who haven't even started. He said TikTok has done a better job allowing small creators to rapidly expand their reach, something Meta is actively trying to cut into.
In terms of overall investment, Mosseri said that Meta has spent "hundreds of millions, maybe a billion or two" during his time at the company supporting the wider creative ecosystem.
That touches everything from incentive payments to the physical infrastructure necessary to power Instagram's AI-backed recommendations.
"We are just seeing more and more power shift from institutions to individuals across the industry," Mosseri testified.
Instagram's struggles around content safety
Susan Musser, the FTC attorney who led Mosseri's questioning, repeatedly questioned the Instagram head over his initial concerns about how the app was ensuring the safety of its content.
Mosser pointed to an email from October 19, 2018, less than a month after Mosseri became head of the app, in which he said that Facebook was not investing enough in Instagram's Well-being team.
"I think we're underinvested in Well-being and were, until recently, the resources we do have are underleveraged," Mosseri wrote to someone whose full name was redacted. The initial email the person wrote was titled, "need to prioritize integrity efforts over growth — we must fight fakes."
An internal Facebook document also showed that Instagram had significantly fewer engineers devoted to well-being than the main app. According to the 2018 summary, Instagram had 40 engineers dedicated to doing such work. Facebook had 900.
Meta lawyer Aaron Panner later asked Mosseri if Meta employees typically received everything they requested.
