I think I agree with Kevin. It seems to be more like listening to a podcast. You are listening in on folks' conversations. While fun and interesting, it may not be as social as you want it. Of course, you could start your own branch and invite some friends to do it with you, but adoption is the key there. If no one is going to use it, it lames up your experience.
Obviously, with most of those services Matt listed above, it's all about the reward. Virtual 'badges' for the most part. And it's a huge hit. Right now. Innovating past that will be the key.
I still do like G+. But, I also get kind of bummed that I'm the only one that I know on some kind of personal level on there. Also, it's mostly self-promotion and not a whole lot else.