Antitrust is about anticompetitiveness. Big tech and especially Facebook is in this new situation where they're causing anticompetitiveness not due to supply side control (i.e. you can get other services that compete against Facebook's products). They're causing anticompetitiveness due to demand side control - the social media that people want to use _in practice_ is all controlled by Facebook. Not because Facebook is intrinsically a superior product (e.g. cheaper, quality of content, technical qualities), but due to network effects and acquisitions of network effect through Instagram and Whatsapp.
We're in a new situation where new precedent needs to be created in what kind of demand side control is desired, permitted and should be dealt with. Which makes this new territory, and it also means that a defense of "we have decent competitors" shouldn't be a decisive argument on Facebook's behalf.