To be fair, the example OP used was that two independent email servers could still send mail to eachother even if they can’t send mail to gmail. I do feel like social networking has a little bit of an advantage over email there, because email, to be useful, needs to be able to talk to almost anyone you might need to send an email to, those specific users. If a few big instances defederate small instances in that scenario, you basically have to use the big instances because you will most likely need to talk to specific users who are on those big instances at some point. However, in a social network, you want to be able to talk to enough people to have discussions and content, but it doesn’t matter as much if you can talk to any specific user or specific account, so it’s much more viable to have a smaller network of independent instances that still functions if cut off from the big ones, as long as they can collectively retain enough users to be interesting.