The main reason people use facebook is to keep up with their family and friends. That's it. You'll change very few minds on it.
They are not there to learn, have their deeply held beliefs challenged, world view shattered, mind blown, politics questioned or to debate anything with any significance. If you believe otherwise you'll find yourself unfriended rather quickly and your chances of waking them up significantly reduced.
So can FB still help spread the message? Yes but indirectly.
If you do use facebook, use it to strengthen relationships, friendships and trust within your social circle. Later on when liberty topics come up, naturally, they'll be more apt to trust and listen to your take on the issue and respect your opinion. Don't fire hose them with information, just enough to wet their appetite to make them curious. Give them something to think about. Keep it lite, especially on facebook. It's not the place for serious discussion and can potentially embarrass them in front of their social circle, even if they might agree with you. Be considerate. It goes a long way. If they want to know more they'll ask you or talk to you about it in real life or through email. If not they may not be ready yet. Let it simmer. They'll come around.