I have been using Facebook for a couple of years and I am quite attached to it as a way of (a) keeping up to date with friends and (b) reconnecting with friends I haven't seen for a while. It has been particularly useful as my family and I moved from Kent to Berkshire about 3 years ago (how time flies). However, despite being a fan of Facebook, it is something I keep very definitely in the realm of the personal, rather than anything work related. I do have colleagues I am friends with, but our interaction is informal and rarely mentions work.

As for the other suggested online networks, I have engaged with a few via Twitter, but others I need to investigate further. Pininterest, for example, I hadn't really twigged what it was, but when I looked at the site I realised I have seen it used to group images together like an electronic mood board. Again something I might try out over the summer - maybe as online extension of our displays.
Overall, I am a fan of online networking. I think it opens us up to so many more opportunities than we could hope to be part of without them; and I think Reid Hoffman's "Facebook is the backyard BBQ; LinkedIn is the office" quote is a brilliant way explain the differnce between the range of sites available.