I told my oldest two boys when they asked for one (then aged 14 and 12) that they needed to write me a proposal outlining why they wanted a FB account and what they saw as their responsibilities should they get one.
That was too much hard work so I managed to put it off for another 6 months at cause they are lazy buggars

Master C was the first to crack and wrote up his proposal which we then added rules to, to make it into a contract. Rules such as, had to be friends with parents, parents have passwords, only one account to be had - stops having the secret naughty one that Mum and Dad wouldnt like, no adding random people, be mindful of what you are posting, comments etc.
Master K has since done his too with the same rules. We did have a few issues with not logging out properly at friends houses so been hacked which resulted in losing access for a week. I dont appreciate having how much my son loves penis plastered all over my news feed as im sure his grandmother doesnt either

All in all I dont have a problem with it except for the amount of time they are consumed by it (I have been known to lose a fair bit of time to it myself though) but it is also a fantastic bribing and blackmail tool. Nothing gets a teenager out of bed faster than threatening to take unflattering morning photos and tagging him in them