As far as I know, while account migration is technically possible it isnt implemented yet, so if a server decicdes to shut up shop due to lack of funds your account there is lost and only federated content on other instances will be saved.

So once migration of user accounts and communities is implemented we should be able to easily survive waves of server closures.

I do worry that it will eventually coalesce into a couple of ultra-large servers and lose a lot of it's decentralisation though. gotta spread out!