Mastodon, and in general, the microblogging side of the Fediverse, is much much more varied than Lemmy or kbin, and your choice of server actually doesn't matter much because there are so many, and in several different interfaces and flavors. There's Misskey, Calckey, Pleroma, Akkoma and many others that I honestly consider better than just Mastodon to interface with the Fediverse at large. And, like I said, they all talk to each other just fine. I'm personally using a few Calckey accounts, check out their .org website to find a listing of some open registration instances and go take a look inside!