Meta CEO Mark Zuckerberg took the witness stand Monday as his social media company faces accusations by the Federal Trade Commission of creating a monopoly.

Zuckerberg’s testimony came immediately after Meta, the parent company of Facebook, Instagram and WhatsApp, and the FTC presented their openingarguments in the US District Court of the District of Columbia.

It’s the first of what is expected to be two days of testimony for Zuckerberg, who will seek to defend two of his company’s most important acquisitions — Instagram and WhatsApp — in the face of accusations from the FTC that the two platforms helped make Meta an illegal “social network monopoly.”

And although Zuckerberg is no stranger to defending his company, the stakes in this case may be higher than ever before. If the FTC wins, Meta could be forced to break itself apart and spin off WhatsApp and Instagram, which would upend the company’s core digital advertising business and reshape the broader social media ecosystem.

Meta claims to have 3.3 billion daily users and applies that count as a core selling point of its ad business, which last year alone reported more than $160 billion in revenue.

The US government argued that the acquisitions were intended to prevent Meta from having to compete with nascent, would-be challengers by buying them instead. FTC lawyer Daniel Matheson argued repeatedly during opening statements that “consumers do not have reasonable alternatives” to Meta’s platforms.

Lawyers for Meta argued that its platforms have plenty of competition in the social media space and that regulators approved the purchases years ago when they were made.

Matheson began questioning Zuckerberg about the transformation of Facebook from a platform designed to facilitate connections between friends and family to one focused more on showing users interesting third-party content, including the launch of features like the news feed and groups.

“It’s the case that over time, the ‘interest’ part of that has gotten built out more than the ‘friend’ part,” Zuckerberg said. “(Users are) connected to a lot more groups and other kinds of things. The ‘friend’ part has gone down quite a bit but it’s still something we care about.”

Zuckerberg also said that messaging is “symbiotic” to Facebook’s larger offerings, given it allows users to share content they find with friends, after Matheson asked if he considered messaging to be a “complement” to the platform’s core services.

That background discussion could be key to how the FTC defines the “market” from which Meta dominates with its platforms.

Zuckerberg delivered his testimony in a monotone voice, reading back old posts and emails he’d sent about the company’s evolution and his thinking ahead of the Instagram acquisition at the prompting of the FTC’s Matheson.

In a September 2011 email — about seven months before Meta acquired Instagram — Zuckerberg told several other executives that “mobile photos … will increasingly be the future of photos” and that Instagram had become “a large and viable competitor” in that space. In that same email thread, Zuckerberg raised concerns that Google could instead buy Instagram, Matheson said.

In a separate internal comment in February 2012, Zuckerberg said Facebook was “very behind in both functionality and brand on how one of the core use cases of Facebook will evolve in the mobile world, which is really scary and why we might want to consider paying a lot of money for (Instagram),” according to evidence presented in court.

Separately,Zuckerberg conceded that in one 2022 email exchange with Chief Product Officer Chris Cox and Facebook President Tom Alison, he was, as Matheson put it, “discussing strategies that Meta might employ to ensure there’s a vision for Facebook in light of concern for cultural relevance,” — a reference to Facebook’s declining popularity compared to Instagram and third-party platforms like TikTok. “That’s generally a good summary,” Zuckerberg said.

–This story has been updated with additional details and context.