@thomas well popularity aside (compared to masto at least) the amount of Reddit alts is approaching the Twitter alts now (just much younger):
Lemmy
Kbin
Mbin (fork)
SubLinks
Piefed
Azorius
(Plus some I prob forgot)
And if forums count, NodeBB, with discourse coming (?) #threadiverse#fediverse
I just attended the "ActivityPub Based Forums and Threaded Discussions" call, and I really like their work on standardising the way, how forums federate.