If a company like Google struggles to launch a new social network with their existing huge user base, you know its hard. Yes social networks and platforms do die, like MySpace and RIMM, and new ones burst in to prominence like Pinterest or Android, but for every new network or platform that succeeds, hundreds or more probably thousands fail.
Small third party developers have strong incentives to put up with the risks involved in using API's provided by dangerous companies like Twitter, Facebook and Apple because they have to if they want to get access to all the users those companies own.
Google failed in part because they never really communicated what Google+ is. I still don't really grok what it's for or why I should use it, and that's why I don't. They really just didn't produce a compelling product, brand it well, or market it effectively. For whatever reason, Google's marketers doesn't appear to be of the same caliber as their engineers.
Someone will pull Twitter's pants down and at the same time eat their lunch. I love Twitter, but I can't wait to see what cool thing replaces them.
Uh, its for pretty much the same thing Facebook is for, except you can do lists and make them public or private, you seriously didn't get that?
Or did you just not use it because the people in your network weren't using it. If your friends are already on Facebook there is almost no reason for you to migrate to a new social network where your friends, or chosen thought leaders, aren't active. Maybe you will tack it on as a 3rd, 4th or 5th social network where it will mostly just consume more of your time.
As I said, yes network do rise and fall but it is enormously difficult to get that ball rolling and keep it rolling. FriendFeed was an awesome social network, with all star founders, but it never gained that critical mass, the founders gave up believing they could displace Facebook so they sold it down the river... to Facebook.