@maria PT is the only other one I use myself right now. Some other worthy ones, which I do not personally use but do look promising are https://pixelfed.social (federated Instagram alternative), https://write.as, (federated Medium/blogging), https://joinplu.me, (also federated blogging) & https://funkwhale.audio, (federated music sharing/streaming).
I am also writing an #ActivityPub server myself, fediQuest, which fills the role of a federated Quora/SO alternative, but no site yet.
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