I agree that there is often little postive value in the interactions mediated by social networking sites. However, "often" is the key word. There are times when taking part in social media can be categorically beneficial, even if those times are rare. To me, this means that social networking sites are not a complete waste of time.
If kids weren't wasting time on facebook, would they be spending their time productively in other ways? I think this is question points to the relative neutrality of the social media trend.
The dangers posed by social media that you mention are real, but I agree with the suggestion above that education is the answer to those dangers. We probably don't need to shut down facebook in order to keep kids safe.