Here's a quick little rule I did up. All it will do is get rid of the login box on the facebook.com page. I'm one of 7 people left who don't have a facebook account so you'll have to figure out all the other places to login on your own. This rule should give you the skeleton to figure that out though. You basically are going to remove the elements that allow people to log in to facebook from the page. It's probably possible to do without Firebug in Firefox but it sure makes it easy and i wouldn't want to figure it out any other way. Good luck.
Here's a good little read on it as well:
For the login, I think all you need to do is block facebook.com/login.php.
@adabicee Thanks for sharing. We're still at 6.8.x though so I can't import your script. I'll try to sift through it though to gather the key points.
@Jon I believe we tried that before, but it didn't work since we haven't enabled SSL scanner yet so we can't block https://www.facebook.com/login.php yet.