Just as teen drivers in training need an experienced driver in the car, parents need to be "in the car" where teens' online social lives are concerned. In the same way that we ask them, "Who are you going out with tonight?" or "Who else is going to the concert or party?" we also need to ask them who they're spending time with online.
Remember that it doesn't help a student driver to have someone in the car who is petrified by teen fatality statistics and has buried the keys in the backyard. It just helps to have someone in the side seat, offering guidance and pointers along the way.