I think federation with Meta will improve Mastodon, and doesn't affect Lemmy too much. Threads users will be able to post to our groups, but the discussion will be mostly within communities on existing servers.

The worry is if the bulk of discussion happens in Meta's space. Yes, people will feel like they are missing out if they are on a Masto server defederated with Meta, but there is enough activity from people outside of that to be engaging. On the Lemmy side, (hypothetically) if Facebook Groups were to become like Lemmy communities, I'd be very concerned that most of the discussion would move away from places like here on lemmy.world and other cool servers to Meta's. Then by the time Meta decides to leave or do something stupid then people will not have a place to go to.