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 in the US District Court of the District of Columbia came immediately after Meta — the parent company of Facebook, Instagram and WhatsApp — and the FTC presented their opening arguments.

The US government accused Meta of illegally building a “social networking monopoly” by acquiring would-be competitor platforms WhatsApp and Instagram. During Monday’s testimony, a lawyer for the FTC argued repeatedly that “consumers do not have reasonable alternatives.”

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.

For Zuckerberg, the stakes are high. If the FTC wins the case, Meta could be forced to spin off WhatsApp and Instagram, which would upend the company’s core advertising business and reshape the broader social media ecosystem.

–This is a developing story. It will be updated.