Discussion forums are, socially, a very different thing than hanging out with friends, even though they both involve people and saying things. That's what I think HN collectively misses when talking about Google+ or Facebook; they think those are Internet Discussion Forums when they are actually something for closer friends.
You have to understand that very few people are willing to write this much on the Internet for public consumption by total strangers -- we are very unusual people, and most online services aren't designed for us. That's why we're typing hundreds of words into a Web 1.0 GUI written in a custom programming language, rather than using Facebook or G+ or Snapchat or whatever.