While I can appreciate the overt "usefulness" of a mass network of "Friends", I find too many people in my generation RELY on facebook to maintain friendships and networking. Call me crazy, but I prefer a good old fashioned hand shake and meeting for a beer then updating my "status" ever ten minutes. I tried the facebook thing for a while, and was swamped with people pestering me that, quite frankly, wanted nothing to do with (morons from high school, people from training command that I never want to see again, etc)
Best thing I can think of about is the photo sharing capability.
case in point, I was going to try out for a band, and the guy wanted my last name. I said "ok" and gave it to him. He said he spent 2 hours searching on facebook for me to see what I looked like (I dont have a facebook account). Would have been REAL simple to ask for a picture. Its leading towards some kind of "cyber stalking" which I find creepy. My story is VERY mild, but I know that it gets super creepy for some people.