By open do you mean not centralised? I don't get the significance of big S social media. Functionally how would big S improve on group chats?
Social media has two functions: chat (within groups/topics/...) and discovery (of groups/topics/...). So unless we rely only on IRL discovery, we need a way to do discovery online.
Once you start algorithmically shoving content to people you have to start worrying about spam, trolling, politics, copyright, and all kinds of issues. The best discovery system is friends sharing chat invite links to other friends who are interested.
The only times on the internet I've felt part of a community was on old web forums.
I used some of those similar web type services for discovery in the past but they became shit fast or shut down. HN is the nearest I can find that surfaces stuff I'm interested in. Social media might have stuff on it but I'm unwilling to waste my time trying to find it