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?

what i find ironic...
guys always seem to fall for the pretty
girls..
the pretty hair..the pretty face ..the pretty eyes..etc..but what thats all they have going for them..
the personality is nothing but plastic.
Eam not saying go after the fugly's..em saying ..before committing yourself to a relationship ..find out more about them first.
Hang with them for awhile and watch how they react to certain things that you do..have a mirror handy too so you can see the facials when they think you aint looking.
There is such a thing as freindship/companionship 1st..the others will fall into place later on.
There is somebody out there with your name tatooed on thier heart..
you just havent found her yet..!!

Girlfriends are hostages to fortune.

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just forget them.

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^^

Short, sweet, and oh-so-true.

The worst thing you can do is waste your life over a girl. I got dumped by this amazing girl last year, and it took me until very recently to realize I was wasting my life and energy trying to get back with her. I sent her this heartfelt email [cause she wouldnt talk to me in person], and she sent it back, basically with a big "Fuck you". That's when I realized I was wasting my time and energy on a total bitch, and moved on. Sure, it's a completly different situation, but the message is the same: Dont waste your time!

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You're pregnant!:D