[LegallyInsane]

I guess that I'm pretty late to this thread, but I'll state my opinion anyways.

At first, I assumed that this guy probably sent her a Friend Request and she accepted to it. I thought about it, and thought that since he knew that she was 'in a relationship', he'd probably want to try and screw things up, or make one of you jealous.

But . . .
You also said that your girlfriend hasn't called you and that you haven't seen her. That's a bit odd. If you two were actually as great as you thought, she wouldn't do that. If she has a cell phone, I advise you to call, and if she doesn't answer, you should definately leave a voicemail for her to listen to later.
If she never calls back, or never contacts you in any way, I'd just leave it be. It'd probably mean that the relationship was over.

I know it may be hard to handle, if that's the case, but you have to stay strong. Don't do anything stupid. =]

How long have you two been together anyways?