Two other reasons Facebook really doesn't work, at least for me: a) the UI is not that of a proper forum or chat (small amount of characters per row, lack of controls for embedding content including quoting, content burried in between a ton of no-related content, etc). Actually, in my opinion the UI is abysmal when it comes to conveying information which is more than a picture of video or few words. b) when you're not in a closed group there's virtually an unlimited amount of people you don't even know who ca chime in and want to present their idea (which is almost like you're trying to have a serious discussion in real life and there's constantly a bunch of drunken people interrupting you st start spitting out whatever is on their mind).
It just does not work for me for having proper discussions (it does have a ton of other features which do work great, but not that). Reddit is sort of ok, if you find the right community. And many of the forums I already visited before there even was a Reddit or Facebook stil exist nowadays and some of them are even more alive than all those years ago. Most of them are specific (eg. one sport, one music style), the general ones are mostly gone.