I think Mastodon is even more dangerous than Twitter/FB in reinforcing filter bubbles.
Consider: There are various federated servers, often with some shared interest/allegiance. The operators can disconnect you from other federated servers they consider untoward. You now have to choose the filter bubble you most actively want to be associated with, and hearing dissent can become technically impossible.
Someone supporting Russia and someone supporting Ukraine in the current war may not be allowed to talk to each other about model trains, because their bubbles blocked each other.
I think this is scary. It's halfway between centralisation (Twitter) and decentralisation (e.g. Scuttlebutt) and thus creates an uncanny valley.
That's not what filter bubble means. https://en.wikipedia.org/wiki/Filter_bubble
But even in the more generic sense -- they can't disconnect you, they can disconnect the account you have on their server -- and it's common practice for people to have alts on different ones just for different topics they post about.
Doesn't that defeat the point of federation?
Not really? Having alts can be a bit like how people have NSFW alts on Twitter (or, for artists, an art account and a personal one), except if you put them on different instances you can find a local timeline that's more relevant to the kind of thing you're posting.
Some people (notably, it seems, Eugen) think "the point" of federation is to provide a seamless Twitter-like global experience that just happens to be hosted modularly. A lot of the current site culture prefers to consider each instance like a little neighborhood, with federation augmenting a local-first experience. (Conflict over product vision ensues.)
But really, no matter what you're looking for, the vast majority of the time you can just use whatever instance and it's fine -- mod drama whole-instance-blocks get a lot of attention, but it's way more of a problem in theory than in practice (especially now you can move your followers if you move instances).