For some odd reason I thought this thread was going to be about aliens.
Edit: Hah, I'm not the only one!
On topic though, I had a friend who was in this exact situation recently. If you think she's worth talking to you should go for it. While I wouldn't advise making a Facebook account just to talk to one person (actually I wouldn't advise making a facebook account at all), if it's the only way you have of contacting her give it a shot.
My friend in the similar situation was given the facebook information of a woman one of his friends met and thought he would like. It took him 3 weeks to get up the nerve to talk to her as he thought it would be creepy/intrusive to start up a conversation on facebook without having met her. She had no problem with him introducing himself via facebook, and now they hang out all the time.
Either way, you have nothing to lose if you think she's worth knowing. You have talked to her in person, so it's not as thought you're entirely a complete stranger. And if things do go south, which is inevitably a possibility, at least you won't be left wondering 'what if'.