Good for you, Colin!
I have found since quitting Facebook that I don't miss it at all. One additional reason that I'm trying to piece together for myself and for my study (I'm going to write a paper on this topic) is that in some way Facebook and other "social networking" sites create a false reality that keeps the user plugged in to the detriment of their real social and personal lives. This may not always happen--as Colin pointed out, some people have the self-discipline to put Facebook in its proper place, as a tool to be used sparingly and only as needed. However, what has made FB so fun and appealing is their very tendency to allow escapism to creep in to your life. I need to think about this more, but these are my thoughts at present.