Anything can be a cause for concern with adolescents because just about anything can be used in bad ways. So yes, Facebook and other such sites can be a cause for concern. However, that is not to say that they are all bad.
The major cause for concern would be that Facebook offers too many chances for cyberbullying and for teens to get themselves in trouble by posting inappropriate statements or pictures. The internet gives us some sense of anonymity or, at the very least, it helps to lower the inhibitions we might have in personal interactions. This can lead to things like cyberbullying which has led to a number of suicides among adolescents.
However, this does not mean these sites should be banned, just that parents should help their adolescents to understand what sorts of behaviors are and are not acceptable on such sites.