FTC does in fact need to show (directly or through indirect evidence) that Meta has monopoly power in a relevant market and that it abused that power in order to win a Section 2 case.
If the relevant market ends up including TikTok or YouTube, FTC will be unable to make that showing.
I know the company is quite unpopular, but from an objective legal standpoint I don't see how you can make an antitrust/anticompetitive argument here.