Most of the old niche groups (IRC groups, imageboards, forums, etc) I used to spend time on just migrated into private spaces once social media became a de facto public space on the internet. These places usually live as Discord guilds, Matrix spaces, Telegram group chats, private fora, Patreon groups, alternate networks (such as: mailing lists, Gemini, Freenet, Urbit, etc.)

I also feel like all this noise about Twitter is just the Twitter early adopters realizing what the old IRC and imageboard users knew long ago: social media is a semi-public space now that everyone is online.