Privacy and anonymity are two very different things. Meta et al very much tie your real world ID to your online persona plus track, analyse, store and sell everything you do on their platforms (and possibly what other sites and apps you use).

Your posts on Facebook can be set to "private" (only Meta and you/your friends see them), but they're never anonymous. Eg if you start saying unwanted stuff if you live under an oppressive government, you will have IRL trouble.

Your post on Fediverse can be public (everyone can see them), but anonymous (no-one can tie your IRL identity to your online persona). Eg you can make an account here and say things about your oppressive government and if you use basic anonymity tools (VPN-s, e-mail aliases etc) you cannot easily be tracked down.

You can also be private and anonymous, eg tipping off journalists about stuff going on under your oppressive government by using anonymous, E2E encrypted message service like Briar. That way no-one can eavesdrop on your comms and only you know your real identity.