Why doesn't this happen on Facebook or Twitter? Here's what I think:
For most people, Facebook is just the wrong context for intellectual discussions.
Similarly, Twitter's medium and user interface are too limiting to have quality discussions since there is no centralized concept of a post. It's just a sea of tweets, retweets and reply tweets that you have to track if you want to follow the discussion.
A lot of my real-world friends are on G+ but that's really only an ancillary benefit to me because I can direct share things to my friends in 1-click from anywhere on the web (e.g. with the +1 Chrome extension). G+ would still have value to me without them, honestly.