To answer your original question, I use Facebook for communication with my friends and low-level awareness of what old acquaintances are doing, and HN for tech news. HN won't help me meet up with my friends to watch a movie, and my Facebook feed rarely has good tech news. G+ has a bare handful of my friends, none of whom live in the same city now, and I haven't found sufficient interesting public content to make it worth my time (though I haven't seriously looked in about a year). Once a week when I call my parents (Hangouts are awesome for that, I can get my brother in at the same time), I'll scan through the half dozen posts in my feed for the previous week.
Also, I stopped checking G+ daily when they started putting up that annoying "find your friends" banner half the time you hit the site, which was way too much friction for the paucity of content in my G+ feed.
I think people decided that Social Network must be equivalent to "People I know, who know me," and Facebook is great for that. If you want to add in "people I used to work with," then LinkedIn is fine, too.
But there's this interesting layer, for me, of other people that are interesting to talk to:
People who are willing to identify themselves publicly with their real name, and who are interested in the same things I'm interested in.
By joining Communities of like-minded people, it's very similar to subscribing to a sub-reddit. Except I personally see way less spam and trolling.
And I can mix that in, with the streams of other interesting people who are on G+.
Facebook != Hacker News != Google+
They're different. The reasons why most people SAY they don't use Google+ is because it's not the same thing as Facebook. I think they're wrong to have even thought of it that way.
I took your recommendation and looked through Google+ for communities that interested me, and didn't see anything promising, but the idea does have some potential - I do follow Linus and a few authors, and would probably check Google+ more often if I could find more interesting content, but I haven't found a killer feed to make it part of my daily routine yet.
I suppose different people use these sites in different ways, though, and I might check back periodically, but for now Google+ is just a Skype replacement for me.
Also, I stopped checking G+ daily when they started putting up that annoying "find your friends" banner half the time you hit the site, which was way too much friction for the paucity of content in my G+ feed.