Some time back, I queried the value of social sites such as Facebook, but for the moment at least I'm won over. I do like this wheel of friends, displaying my Facebook friends and how they are connected among themselves (click on the image to get a greatly enlarged and legible version). Only a small number are not connected with any others in the circle.
I've found that Facebook actually is quite good for networking, as long as the people you want to network with are actually there and you already have at least some connection with them, however tenuous. Also, it's almost too tempting as a place to hang out with people whom you simply like.
What's notable is that most of my very close friends are not there (but there are some exceptions ... you know who you are, I hope; kisses to you). Likewise the colleagues with whom I work most closely, to the extent that it's a different group, are generally not there (but again with exceptions; the transhumanist/technoprogressive push is especially well-represented on Facebook in general, and within my wheel).
Overall, it's worthwhile so far - if you're close to me as a friend, or colleague, or both, do think about joining and kind of hooking up with me.