I post on Reddit. I think that's the only "general online discussion forum" I use if you don't consider Facebook (FB Groups are basically a collection of topic-specific forums, if you wish to consider it as such)
I miss the times when more theme-specific forums like this existed. A lot of forums have vanished and been replaced by Facebook Groups. Which is in my opinion just not the same, and most of those groups lack the community spirit of a forum.
I used to co-moderate forums in the past, such as a forum on Belgian (and international) football/soccer, which was a great experience. I also co-moderated a political forum, which was a less good experience and which I gave up after a while due to too many foul language and personal insults between members. But the football/soccer forum was very rewarding and a great community spirit existed there, despite all members being fans of different teams. I miss that kind of spirit on Facebook Groups, but I guess the evolution cannot be reversed and forums will become sparser and sparser.