Internet comments sections where the comments are frequently insightful/articulate and not too full of obvious-garbage [or at least the top comments, given the comment-sorting algorithm]:
LessWrong and EA Forum
Daily Nous
Stack Exchange [top comments only] (not asserting that it's reliable)
Some subreddits?
Where else?
Are there blogposts on adjacent stuff, like why some internet [comments sections / communities-qua-places-for-truthseeking] are better than others? Besides Well-Kept Gardens Die By Pacifism.
Arxiv is basically one huge, glacially slow internet comment section, where you reply to an article by citing it. It’s more interactive than it looks- most early career researchers are set up to get a ping whenever they are cited.
10habryka
Some other places off the top of my head:
* ACX often has good discussion in the comments (but the lack of voting makes it quite hard to find them)
* AskHistorian subreddit
* There definitely exist good Twitter conversations but no good way of finding them. I do think following Gwern, Eliezer, Ajeya, Emmett and a few others on Twitter does tend to surface high-quality discussion
* Hacker News sometimes has good discussion, especially when the authors of a linked article show up