As it's a laptop, your stepdaughter will probably take it out of the house and use it in various locations, so all these suggestions about setting up filtering in the router strike me as rather misguided.
The first question is, has she got the root password? If so, she or her friends will be able to undo anything you do.
If not, editing the hosts file as suggested above is the obvious way to go, but you won't stop her accessing Facebook from a friend's computer, and whatever you think of it (and I refuse to use it too because I think it's pernicious), Facebook is a preferred medium of choice for young people and they expect to communicate through it. You are trying to deny her what her generation considers normal social interaction, and I think you are in danger of alienating her.
Also, remember she will have classmates who can use a livecd to reach root files even if she can't do it using the installed system, and they might even replace the installed pclos with another OS, or if they're clever, a new install of pclos which looks the same to you but lacks your lockdowns.
I suspect those who think you need to educate her to act wisely rather than try to lock her out of the real world have a point.
But I'm beginning to sound like an agony aunt.