I like Facebook. Over the years, I would occasionally ask Mr. Bing or Mr. Google to search for old friends I worked with or grew up with and to no avail. Literally years were spent trying to get in touch with some people.
Then I did two things: I joined Facebook and Linkedin. Within a few months, I not only found those I was looking for, but other friends I had forgotton about.
I think when used with a little common sense, these sites can do great things. Just do not tell your life story and give out personal info and these sites can be of great benefit.