What happens when a forum or medium expands to basically the entire population? Twitter, at least for people who read both popular posts and reply chains that those posts inspire. It makes Eternal September look like a book club full of the world's most articulate English professors sipping tea and politely conversing about complex plot points while taking care not to say things that could be misinterpreted or incorrectly seen as a personal attack on someone else.
When people post good things (not necessarily positive things, but at least negative things that spark good conversations), those posts may or may not inspire other good posts. They probably won't inspire bad posts, worst case is that they don't inspire any responses but that's OK- good posts stand on their own and not everything needs to be a conversation.
Similarly, when people post bad things (not necessarily negative things, just things that spark bad conversations), it will hardly ever inspire good posts in response, but it's very likely to inspire many more bad posts.
So, good posts inspire more good posts, bad posts inspire more bad posts, but IME bad posts are like 10-100x more likely to spark followups. Going back to Usenet, you could have the greatest newsgroup in the world, but it could all collapse due to an influx of shitty posting from a teeny tiny subset of the people reading the group. It's like a virus that infects a forum. The bad posts explode in response to each other, the good posters slowly drift away altogether, and all of a sudden you find yourself in Slashdot or Usenet circa 1999. It's basically nothing but people insulting each other or using bad faith arguments or just writing at great length about how much they think stuff sucks.