I think it's fairly obvious that Facebook's enormous success is at least in part because so many businesses have gotten on board. Even little mom and pop stores have "like us on Facebook" stickers everywhere. Which in turn makes people, even old people and such that aren't usually so tech-savvy, want to check out this Facebook thing. Which makes more businesses want to sign up, and we have a nice feedback loop going.
Yes, people need to be on + for its success, but Google has just taken too long to get the ball rolling. There was a brief surge of interest when all sorts of people were trying it out, but then everyone but tech people simply went back to Facebook where they can play games and get malware and be marketed at.