Well critical mass is the issue here. Social media sites are “fun” only if there are people there posting, talking. So it’s perfectly fine is there are say, 5 fediverse isolated islands that don’t talk to each other as long as each island has critical mass it‘ll be sustainable.

Also the benefit of fediverse is that I don’t need to understand that different communities are on different instances - I just go into search and find communities I want. Yeah some will have funky @names appended to them but I don’t need to understand what that means in order to interact with those communities.