Mastodon is part of the decentralized fediverse, there's a lot more than just mastodon.online there's mastodon.social, and ~10k other servers. I run thebag.social for libertarian/voluntaryist types. Users on thebag.social can (but almost never do) interact with users on mastodon.social but we interact a fair bit with the rest of the network.
NVK (npub1az9…m8y8) used to run bitcoinhackers.org too NunyaBidness (npub1vwy…zugd) used to live over there.
fiatjaf compared the fediverse unfavorably to nostr when he created nostr. https://github.com/nostr-protocol/nostr#the-problem-with-mastodon-and-similar-programs
nostr certainly has advantages over the fediverse (you own your keys and therefore your identity on nostr, on the fediverse you have a user handle that is tied to a particular server, and essentially owned by the server operator, like email) but I still prefer the fediverse so far (it probably doesn't hurt that I own my server and therefore concerns about identity ownership are pretty much moot).