I'm finally starting to find LinkedIn kind of useful (http://micahelliott.blogspot.com/2008/04/sponteneous-recomme...), when the various features are used holistically, and profiles near completeness.

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.