Google Plus is a curious amalgam of Facebook and Twitter but more interestingly this is the same model that Friendfeed pioneered (with far slicker tools: “like” and “real-time feed” anyone).
There’s an incentive to build your REAL social network (a la Facebook) that Google+ is trying to foster with Circles, but at the same time they pollute that atmosphere with the follower model, where people you don’t know jump in with comments that you don’t feel like responding to.
That was the problem Friendfeed faced and that’ll be the problem that Google+ will inevitably encounter.
Now, granted the asymmetric model gains traction and followers fast, the question remains: is it sustainable? Time will tell.