But you can register an account on any node[0], and communicate with anyone else on the entire network. That's the strength of federated protocols.
How well has federation worked out in practice (for other federated, social network related protocols) so far?
As far as I know, federation has only worked for ancient stuff that has nothing to do with social networks, like email and DNS. Basically, it is a part of core functionality and thus can't be co-opted by commercial interests (though GMail has made quite an inroad!).
Until it has proven itself, social federation doesn't really seem like a strength to me. It does sound good in theory! Other people with actual experience are adding their anecdotes which lines up with what I'm trying to say.
Email only works _because_ of big players like GMail. Running your own server spam free and away from black lists is an endless task.
DNS is going a similar way with more and more ISPs resorting to hijacking DNS lookups for all sorts of nefarious reasons. This protocol seriously need a broadly embraced signature system to validate origin.