It sounds like a hard thing, but in case of XMPP "rebuilding your social graph again" is very easy - it's just a matter of importing your roster and sending authorization requests where needed. Could be, and probably already is, easily automated with some user friendly tool.
If the solution to censorship is to constantly switch to new hosts, it would be even easier to do this via a VPN (which wouldn't require you to rebuild your social graph at all, unlike a federated endpoint switch).
If the more straightforward solution (VPN) isn't a panacea for censorship, then federation isn't either.
Of course it's not a "panacea". It just makes some situations easier to handle, including server operator going rogue or broken by government. It doesn't magically provide answer to everything, but it's definitely an improvement when compared to purely centralized networks.
