There’s a bit more on Mastodon then Lemmy - https://docs.joinmastodon.org/user/moving/#migration has the details. But not being able to move posts is a big limitation. And even the functionality that’s implemented has some unpleasant surprises – see https://erinkissane.com/notes-from-a-mastodon-migration
Also SmokeSignal events https://smokesignal.events/
I saw a talk by the peole working on FrontPage last week, they use the Bluesky Relay, filter it down to only see the posts they’re interested it, and have their own AppView. It doesn’t yet have the same kind of intereoperability with Bluesky that Lemmy does with (say) Mastodon; FrontPage posts are only visible to FrontPage. But, there are discussions on how to get beyond that.