I joined thanks to a couple of friends. You cannot join unless someone invites you who already is on Google+. That right there is the win that will knock everyone out of the water. I will elaborate.
Think back to 2004. Gmail was just launching from Google. Everyone wanted an account, but you had to know someone to get one. I was working in Ottawa at the time and people were bribing others in the office because they had Gmail-invites. I believe I bought a guy a large coffee from the Second Cup in the office concourse. I already had an account. In fact I had a web domain and my OWN email account on my own server. But I wanted a Gmail account. I probably would have bought the guy two coffees. There is the crux of it all… Demand.
Gmail was in demand because you couldn’t just go out and get it. People didn’t know if Gmail was good, they just wanted it. Marketing 101.
When Google came out with “Buzz” they just gave it to them and no one cared. Now “buzz” has been rebranded “+” and tweeked, and people want it; They cannot just sign up for it and that creates the demand.
Plus could be a turd. But marketed right even a turd can smell like a rose.