Nobody tries to have these kinds of discussions on Facebook because it's obviously the wrong place (my mom doesn't care what I think of bit-shifting). Disqus is fine because it's in the context of the blog post. People have to seek it out. Twitter is fine because conversations are not globally visible, they are filtered by @replies to relevant parties.

Google Plus was seeded with Google employees and they are the most enthusiastic users so it is filled with these kinds of discussions.