Earlier quoted context omitted.
One of the few times I’ve heard a counter to this was an old fisherman who took us out on a boat here in Melbourne, Australia. Proper bloke, skin like leather from the sun, done nothing but run fishing trips his whole life. He was lamenting the loss of a particular forum with one of the other guys. “All just on Facebook now”, he said, and he said it with sadness. He missed that old forum.
Actually that makes a lot of sense, it's not that we need to have RSS replace Facebook, we need a resurgance of forums to take back from Facebook. Perhaps if forums could talk to each other you'd have an experience more like mastodon, but in concepts that more people understand. Something like an open source Stack Exchange network. So you can log-in to one of them but then easily create an account using the credentia…
https://insightful.demo.talkyard.io/-7/how-do-i-get-myself-o...
2. Single sign-on, and (as you wrote), "using the credentials of the original forum":
What about using Scuttlebut's identities? Everyone has his/her own ed25519 key pair. "The public key is used as the identifier", see: http://scuttlebot.io/more/protocols/secure-scuttlebutt.html. These can be created in a decentralized manner. The keys are really long, but maybe, in a specific forum, one wouldn't need to show the whole key. One could instead show a forum-local-@username + the shortest unique key prefix (unique in that forum).