Why does he only talk to me at school?

I know a guy from school since 5th grade. He's in my homeroom and we talk at school a lot. It's the end of the school year so we went back to homeroom for 2 hours at the end of the day. We sat together and looked at my yearbook as the others watched the movie. We were laughing and stuff and having a good time. He was definetly flirting with me and I sorta like him now. He said he would date anybody if he likes their personality and he even told me he liked mine. I recently made my facebook account again but when I try to talk to him he wont reply to anything I say. THing is he's really popular so he is busy I guess but everytime he's on I might say "hey" or "how was ur day" or something. But EVERYTIME he wont reply. He seems interested at school but wants nothing to do with me when on facebook. What's with this guy?