I am on Facebook and quite happy to be there -- it serves me professionally and socially.
The other day I jumped into a conversation with several senior executives -- I'd overheard one of them state, "Facebook is not useful for business people. It's mostly for young folks..." I emphatically replied, "Facebook gives me back my village!"
Just to be straight up, I am not paid by Facebook nor do I know anyone that works there, but what I am finding there makes a lot of social sense.
I certainly Twitter, I'm on Linkedin, I blog (duh!) and have profile pages on a few other sites around the Internet, and the common thread among these is they are all confined to a narrow professional and/or personal niche. None of them show the "complete me".
On Facebook my social and professional life cross over and people can see a more complete picture of whom I am -- which I am happy for them to know.
Look at it this way; long ago when we lived in villages and small towns everyone knew everything about everyone. It did not matter what our profession was or what friends we had -- in a village there is no place to hide. Trust was built between people that could know all there is to know. I like that.
I want people to know all about me so that I can cross the bridge between knowing and trusting as quickly as possible. And on Facebook that bridge is being built. So I update, send out "what's on your mind" posts, join groups and participate in the activities of my village.
I would encourage every professional to get engaged and use the tools we are offered to get connected, be known, and be trusted. It WILL work for you!