That is not the same as moving your old account to a new instance. If your old server actually dies, your old account data is just gone. And if it dies before you get a chance to migrate, then you're really out of luck.
I just don't understand why people feel such a strong desire to cling to the federated servers model when these problems are solved in decentralized protocols already...
The original question to which I replied were not talking about the case where the server goes offline. If Facebook were to go offline, you'll have the same problem, only worse.
As for your final remark, I really can't see a good way to handle such a situation at all. And I've never heard about any protocol that can handle it. What protocol are you talking about?
I just don't understand why people feel such a strong desire to cling to the federated servers model when these problems are solved in decentralized protocols already...