I've tried that on my office PC way back when I first installed Windows7 and it didn't work very effectively.
If I just blocked facebook.com it would work but if someone clicked a link that was say: facebook.com/username/comments it would let them in. Once they were in they could browse their profile and have full access.
In other words there appears to be countless ways someone can get into facebook without using the "front door".
I've also used free trials which worked okay but then I'm the d!#k for using apps to block my family and then they all hate me.
I just want something non-intrusive, something I can play dumb and be like, "I dunno why facebook isn't working?"