There's an important difference, though, especially with Lemmy. You used XMPP to communicate with particular people. When Google convinced, whatever, 70% of users to use Talk and then slammed the door shut, the smaller instances were no longer viable. People on those instances lost contact with their friends. They aren't going to just chat with whoever else happened to be left outside the walls.

But I don't look for specific people on Reddit, or on Lemmy. Any large-enough instance is fine. Just like people moved from Reddit to Lemmy, they can move from one instance to another. A major rift could drop the quality of the experience, at least for a while, but the instances would still be viable. They're not suddenly useless the way an isolated Jabber server was.