Reasonable concern, or overreaction?

[Now, Bring Me That Horizon...]

Right. Well, my boyfriend and I have been together for about 10 months. I'm very much in love with him, and I know he loves me. However, a few months ago we had an issue. I found out he was lying to me about who he was riding to school with. He claimed it was one of his guy buddies. Turns out, it was a girl he used to have a thing for. This girl has a horrid reputation at school, but that is not what bothered me. It was the sheer fact that he lied to me. I told him that if he had just been honest and told me he was riding with her, I wouldn't have a problem. He swore there was nothing going on, and I even talked to her about it. She said she didn't have any sort of thing for him. Anyway, we patched things up and now we're okay. However, just this morning I was looking at her facebook profile because I was bored and I've never even really seen her. It was a random, spur of the moment thing. Apparently, this girl doesn't post much, because I started seeing things from much earlier this year, and I happened to come across a video my boyfriend had posted on her wall in January. It was a song I know he loves, he's even sung the song to me before. Animals by Neon Trees. He likes to always recite the one line from the song "Take a bite of my heart tonight." And well, he put this on her wall in January, 2 months into our relationship. She commented on it and said "aww. " So, my question is, is it wrong of me to feel hurt and a little betrayed by this? Should I just let it go? Or should I ask him about it? I hate facebook and all the drama it causes. It's stupid. I thought this relationship was so much more mature than that. So please, what should I do?