I don’t have facebook, and I don’t know how it works, but Philip Rosenthal, who does, highly recommends against it, certainly for teens.
The only “story” I know first hand about it was a (now married) friend of mine who closed his account in disgust when girls he had dated asked him to be his “friend”.
Like alcohol, the “anonymity” of sitting in front of a screen reduces inhibitions. Same with texting, etc.