My girlfriend has this friend that won't stay out of our business?

My girlfriend talks to this guy online, he doesn't live anywhere close to her but I do not like him, period. He is this big cocky farmboy and he doesn't like me either. Anyways, one day me and her have a fight and it gets aired on facebook, he immediately jumps and gets in our business. He tells me to leave HER alone and says she doesn't wanna talk to me so butt out. I flipped at this. I really don't like this guy and he's always commenting on her page and I am quite sure what he is up to. She swears he is just a good friend and a big brother figure, but she doesn't know this person, she doesn't live near him, she just knows his facebook persona. I know there is attraction there, for him towards her and she claims to just feel bad for him being he's an overweight goofball. But I feel he is trying to sneak in there and steal her away by trying to prove to her he can be better for her than I can be. And I tried to talk to her about this and she got mad, saying I am just assuming he wants her and they are just friends. What are my options here, because I don't wanna fight with her, she won't listen to me when I say he is after her and I strongly feel he is waiting on the sidelines for me and her to have another fight to play the knight in shining armor to whisk her away from this horrible guy that is me.