I was thinking that federation still leaves unsolved the issue of knowing the single source of truth for identities, and when instances go down, you can't reference anything to know who/where people are.
Couldn't Keybase.io solve this? It could track your identities on these servers, and it would exist outside of any instances or federation, but be the source of identity that would go along with any federated instances you are a part of
Isn't keybase.io just a way of storing public keys? If they go down, and you still have the private key, you can just sign a message saying "No, really! It's me!"
The issue with distributed systems that rely on public keys is the lack of options to turn to if the keys are lost or compromised. And yes, people will lose keys. Systems that rely on keys for identity need to figure out what options people have at that point to re-establish their identity.
Couldn't Keybase.io solve this? It could track your identities on these servers, and it would exist outside of any instances or federation, but be the source of identity that would go along with any federated instances you are a part of