Mastodon: you should randomly pick some person's server to hang out on and trust with your data.
No, of course they aren't registered data controllers.
The law is still a thing (especially in Europe), and companies like Twitter are at least somewhat more accountable to the justice system for misuse in a way that the pseudonymous server admin of your favourite queer polyamorous furry art sharing instance can't. Just saying.
Federation: still bad.