I’ve been using Mastodon since Twitter was taken over by Musk, so I’m not a super long-term user, but I can give my perspective:
Platforms like kbin / lemmy are more like “topic” communities. Like people on kbin create “magazines” and on lemmy they are actually “communities”. From the user perspective, you can just look for these communities you are interested in and sign up to get updates from them.
Platforms like Mastodon are more like you and specific people you like to see content from. So you find people you like to hear from and you follow them to get their updates. They may post on subjects you aren’t interested in but oh well, that’s up to them.
Both formats can produce desirable, tight-knit communities, but they just use different structure. In my opinion, the kbin / lemmy style is more accessible in terms of finding people interested in a specific subject but feels less personal since you are just all there to talk about a specific subject. On Mastodon, when I find people posting content I like, I end up learning more about the random nonsense they are interested in. Like my feed there has a ton of moose pictures now because one person I followed likes to post pictures of moose. I don’t mind seeing them, but I never expected to see so many moose.
TLDR: Mastodon is about following people, kbin / lemmy are about following topics.
quick summary of the most notable instances i’ve read about:
lemmy.ml is run by pro-china folks. there’s some drama surrounding that. it is defederated (fully or partially) from some other instances.
lemmy.world is the biggest lemmy instance. if you want to find something reddit-like, this would probably be the first place to try.
beehaw is a much more private instance that only has a few moderators but has stricter standards on being nice to one another. it ended up having to defederate with lemmy.world early on because it couldn’t handle the moderation load from too much content.
kbin.social happened to pick up a lot of tech type folks for some reason. it was just a side project from one dev so it was not as polished at first and barely survived the incoming users but it’s in a good place now. he’s got a few folks helping him so the future looks bright to me at least.