Ha, I've had the exact opposite experience. I've got a handful of friends who are intelligent and mature enough for nuanced conversation about topics most people have trouble discussing like adults and (not coincidentally, they tend to be the friends I'm closest to). But meatspace is horribly inefficient for getting exposed to a wide variety of ideas; sheer numbers alone make a forum with the right norms almost impossible to beat.

"With the right norms" is the tricky part. There are some fora a couple degrees of separation from SSC that I've had good luck with, but it's really tough to find places like that