Pulling the plug, setting up user accounts etc is great if you want to keep your kids off facebook while at home but they will just start resenting the hell outta you and go off somewhere else to do it. At that point, good luck ever finding out who they are talking to.
You may want to sit down and try to spend some time with the kid and find out why they are posting, what they like, hobbies, friends, aspirations etc. Its not going to stop them from posting, but it may make them more inclined to talk to you about any problems they have rather than vent on facebook plus it will give you some insight into where they may be posting and what they may have up their sleeve.
Doing this might make them more interested in you and maybe make them want to spend some time with you rather then all of it on facebook.
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Milkman - Guardian Bard