If she gave you her facebook that does mean she's interested.
Otherwise, why the fuck would she do that? The only exception to this is if you were directly asking her for her number and instead she gave you her fb. THEN, that's an indicator of disinterest and she's just trying to be nice.
Anyways, just be casual...use plausible deniability. You guys go clubbing in the same area? Perfect!
Shoot her a very brief and to-the-point message.
"Hey thanks for the add...just promise not to stalk me too much

*some sort of playful neg for flavoring*
So I'm hitting up [some club] this friday...come along and (blah blah blah)"
You get the picture. It's VERY casual and laid back. You are her friend and want to let her in on the fun you're going to have. That's it. Do NOT ask her out on a date or to dinner. I guarantee you will lose her this way (if she accepts it would be out of niceness or she's friend zoning you).
Wish you all the best brother. If you have any other questions just PM me.
SEP