Hi, here You've got stats based on almost 4 million analysed domains. You can see that more than 60% of all websites (all scripts, types, languages) use Facebook plugins. What is really interesting - G+ is far, far behind - only 4,71% websites uses this plugin. http://www.statino.com/statistics You've got also othere stats - such as HTML types, encoding, websites speeds there. I hope that it will be useful for someone.
Facebook has millions of users and it's a big pool of niches! However, Google + can be targeted when you have people on circles who are interested in the same niche you're interested!
I've just started using Google+. It might be far behind sites like Twitter and Facebook, but it's growing rapidly and I am just getting into the habit of participating---think I'm enjoying it. But like they say, I have noticed my pages get indexed within minutes now since I've started using Google+
I don't know about the majority of people, but not using Google+ would be a mistake. For one, it's very integrated to everything. For two, Google actually owns it. You know, just the one everyone is hoping would rank them... maybe you're already getting points because you're using their product. And I would assume Google+ is very compatible with Google (as they do come from the same developers), so maybe you're getting indexed faster. Well anyways, you don't even have to choose... just put facebook, twitter, and google+ in your site and be down with it.