I spend about 30 seconds a week on it, so it's not like I'm really active with it.
Linkedin is a slightly different case. It is like a useful business directory.
The next wave will be vertical SN's.(few billion dollar ideas there), and networks with a strong mobile component.
I have started to dump all my entertainment focused social networks. I keep an account on facebook, but only use it to keep track of people. I hate their stupid applications.
Some others (facebook) I just sign up and experiment to learn how the user interactions and features play out. In general, I'm not too interested in being a part of their microcosms. But if massive amounts of users like to spend time using them, we should probably understand how and why.