Good question, and I'm not the one to answer it I'm afraid - I find FaceBook a hideous prospect. My social networking is done face to face, rather than via the web, and although I keep getting emails telling me that this or that person has added me to his/her list of FaceBook friends I have so far found it pretty easy to resist.
One of my daughters was a keen FaceBook fan, but has given it all up - she said she found it addictive, and realised she was wasting far too much time with it.
I find it a little bit odd that anyone who hangs out their flag on Facebook could be worried by some companies knowing a few things about their buying habits. Such information was being stored and shared long before Facebook was dreamed up.