This isn't quite right. GNU social is another version of fediverse software which uses os status. Mastodon implemented os status and then activitypub for federation. I believe the os status support was the reason why mastodon usernames can't be changed despite activitypub supporting it but I may be wrong.

Pleroma is a fork of mastodon that was supposed to be more lightweight and there have been further forks of that such as rebased and akkoma.

Misskey is a separate activitypub project that provides a richer experience including being the first to provide quote posts but is probably the most resource intensive of all of them. There are forks of this as well.