As I understand it, Mastodon doesn't federate like Lemmy/K-Mbin does (and Sublinks & Piefed too!), where in order to follow someone you need to be on the same instance? Or something? Anyway Mastodon needs some work before it could be a viable replacement.

Edit: I did not phrase this well at all. Oh well, it led to an interesting discussion so I'll leave it here for posterity, but it's incorrect. I think the only correct part of the above is that if you try to leave a Mastodon instance, then like a Lemmy one, you can't really take your account with you (only your settings, but people who followed you before will have to now follow you again in the new location, it does not automatically transfer). So it "federates" but it's not "freely transferable" as people were over-selling it to be.