Group chats have kind of taken on this role, haven't they?

As you mentioned, Discord fills that spot for young people, but in general I get the impression people spend most of their time on group chat/private server environments nowadays. Social media is mostly treated as read only, a place you get memes or news from. Maybe there's that one rare friend who actually posts on Twitter or Reddit.

This gets mentioned occasionally, but I'm kind of surprised how little people talk about it, still. All anecdotal for me, of course, but still I find it interesting.