There was a story last year detailing how various state revenue departments were using Facebook to track down tax deliquents.
Why anyone would want to post their entire life for the world to see is beyond me - what good can come from it?
Complete self-centeredness. It is like Twitter -- some people think anyone (or everyone) gives a $hit about the fact he/she is at the doughnut shop.
I keep the tiniest footprint possible in the Interwebz. As FD2003, yeah, I get around. As my RL name, just a few business sites and even then with no personal information.