There were always plenty of forums full of people with only a passing or casual interest. Even outright enormous lunatics - it's just that it always seemed a bit more reasonable overall: Everyone cooperated to not let the place go to shit immediately and the need for huge and unilateral moderator intervention was limited.

I think the large social networks have mostly poisoned a lot of people with harmful memes (the need to shout, weird conspiracy theories, etc) and now that the damage is done it's too late even for the old format. As a society we've moved past being adults online and degenerated straight into childlike behaviour.

The main difference was that on most forums you didn't write to get likes it points and your post wouldn't be drowned out most of the time.