I’m not quite sure I understand what the problem is. I have had Facebook for many years, and I find it to be a wonderful tool. I have met and interacted with people I never would have met in actual life. These interactions have served to challenge my ideas, refine my thoughts, influence others, and enjoy access to a wealth of valuable information familiar to my “friends,” but which I myself would have never dreamed of.
Facebook is a tool. Like all tools (knives, gins, hammers, computers, telescopes, cars, and yes, the CR!) it may be used for bad or for good.
If our children are capable of using the valuable tool for positive and constructive purposes, and are able to identify and resist negative influences (as we hope they do each day walking down the street, going into a store to buy some coffee, or do some research for school in the library or online), then by all means, let them be on Facebook. Let them broaden their knowledge and horizons; let them interact with those that are different then themselves. If they can’t handle the challenge, best to deny them this outlet. Though if we do, we had best be prepared to restrict them from other tempting scenarios like riding the subway or bus, walking down a city street, going to the library, reading a book that we have not pre-approved, meeting new friends, and enjoying some recreational time without big-brother (mother/father) literally looking over their shoulder.