> If I want to have a discussion about subject X I would need to be deep inside echo chamber social media network Y.

To me, this is the crux of your problem. Social media is like a, well, social space like a bar. Not everyone shares the same opinions, but most of the patrons can at least agree enough to not fight each other, which is sort of an echo chamber since yelling something against the grain would get your ask kicked in a bar.

Forums are the place to have a discussion about subject X, since everyone is there to have that discussion. Of course, if you get off topic or snippy the conversation may devolve, but if you stay on topic you can have a nice conversation about subject X.