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.
nostr:npub1az9xj85cmxv8e9j9y80lvqp97crsqdu2fpu3srwthd99qfu9qsgstam8y8 used to run bitcoinhackers.org too nostr:npub1vwymuey3u7mf860ndrkw3r7dz30s0srg6tqmhtjzg7umtm6rn5eq2qzugd 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).