With chat programs, the concern is over monitoring. The US can issue warrants to get your chat records from Messenger/Hangouts. Those warrants are obtained a bit too easily due to FISA/Security Letters, but that's another due process issue/policy issue. With Chinese companies, they don't even need the illusion of warrants or due process.
FB/Twitter/Tiktok are different because they control WHAT you see. None of them show things chronologically. Their algorithms show things that are popular, and the weights they use are trade secrets. Being able to influence ideas takes classic advertising, or even targeted advertising, to an entirely new level. It has an entirely different set of legal and ethical challenges.