What Does It Mean When He Doesn't Reply?

[cheerstothegoodlife]

I met this guy and I really like him and I think he likes me. He's the one who hugged me first, he's the one who added me on facebook. He's so sweet and nice to me. He said himself he would message me on facebook, which he did, and made me so happy and we talked last night.
However he didn't reply to my last message (it was just a really casual message) in 19 hours already. He posted stuff on twitter 6 hours ago though...and he doesn't have school this whole week so he isn't busy or anything.
What does this mean? I want to cry. I really like him and he seemed interested...I'm so sad. And I don't want to message him because I don't want to seem desperate.