At Trial, Mark Zuckerberg of Meta Calls TikTok a Major Competitive Threat

Mark Zuckerberg, the chief executive of Meta, took the witness stand in a landmark antitrust trial for a third day, saying on Wednesday that the video app TikTok has emerged as a serious competitor in social networking.

In a friendly exchange led by lawyers for Meta, Mr. Zuckerberg said that the fast growth of the Chinese-owned app was “probably the highest competitive threat for Instagram and Facebook over the last few years.”

Mr. Zuckerberg’s lawyers were trying to poke holes in the case, Federal Trade Commission v. Meta Platforms, which went to trial on Monday. The F.T.C. has accused the social media company, which was previously known as Facebook, of acquiring Instagram and WhatsApp when they were tiny start-ups in a “buy-or-bury strategy” to snuff out competition. Meta’s core function is connecting friends and family, making Snapchat its only serious social media competitor, the F.T.C. has said.

Mr. Zuckerberg countered during his more than seven hours of testimony so far this week that Meta faces significant competition in the world of social networking, including from TikTok and Apple’s iMessage. On Wednesday, he said Meta’s addition of a short-video feature known as Reels to Instagram and Facebook was in large part a response to TikTok’s rise. Users continue to engage more on TikTok than with his apps, he said.

“TikTok is still bigger than either Facebook or Instagram, and I don’t like it when our competitors do better than us,” Mr. Zuckerberg said.

Judge James E. Boasberg, who is presiding over the case in the U.S. District Court for the District of Columbia, must decide whether Meta broke the law. The government plans to seek a breakup of the company if it wins.