I had an account for about a year, but my best friend and I both closed our accounts at the same time. Her reason: she felt she was being a busybody :P My reason is probably more vague. As somebody said above, I found it hard to be so open to so many people at the same time, it felt like I was reducing my life to the lowest common denominator.
I'm more of a one on one person anyway, so Facebook isn't a good fit for my personality :)