WASHINGTON—Facebook parent Meta Platforms faces a high-stakes trial in Washington starting on Monday on claims it built an illegal social media monopoly by spending billions of dollars to acquire Instagram and WhatsApp, in a case where U.S. antitrust enforcers seek to unwind the deals.

Meta CEO Mark Zuckerberg took the stand on Monday in a historic antitrust trial that could force the tech giant to break off Instagram and WhatsApp, startups it bought more than a decade ago that have since grown into social media powerhouses.