PieFed, Kbin died but lives on in its fork Mbin, Sublinks didn’t pan out but was started (the PHP code is FOSS), nodeBB, and flarum.
You may also occasionally see posts from Friendica or Mastodon that also use the ActivityPub protocol, though the barriers to sharing those with the Threadiverse are somewhat high.
PieFed, Kbin died but lives on in its fork Mbin, Sublinks didn’t pan out but was started (the PHP code is FOSS), nodeBB, and flarum.
You may also occasionally see posts from Friendica or Mastodon that also use the ActivityPub protocol, though the barriers to sharing those with the Threadiverse are somewhat high.