I think with children, or anyone for that matter, teaching them how facebook works is as important as keeping an eye on who they are "friending" and what they are doing. If they go in to social media as a "real" conversation, and don't say anything on FB that you wouldn't say in real life, or that you wouldn't shout out loud in a crowded room then they will be fine.
Social media isn't going away. In fact, who's to say that in 3 or 5 or 10 more years facebook and twitter are extinct (like MySpace) and a new outlet has come up. Learning to live with and use these social media outlets to your advantage rather than your detriment is an important skill.