I wouldn’t ban it. However, I think you should provide a limited amount of time during which they can use it. If they disobey you and spend more time on it (at home where you can monitor) then they are allowed, the punishment should be loss of the next day’s time or something similar. They need to obey you when it comes to how much time they are allowed to spend online. You can, as someone else suggested, turn off the router or limit access to Facebook at home for certain periods of time. You also need to teach them about the ramifications of inappropriate Facebook use. Show them how people have harmed their lives by posting inappropriate pictures and information. You also must be consistent and enforce the rules and any punishments. Otherwise, don’t bother.