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 US Federal Trade Commission’s (FTC) case, which went to trial on Monday. The FTC has accused the social...