We've all been there (unless you haven't in which case I kind of hate you). You meet him, have flirted and there's a spark. There's some minor back-and-forth communication, of the electronic variety: you write, he responds, you write back. Then, nothing. Because you're now social network friends you can see that he's online: and can safely rule out that he has not been hit by a car or kidnapped by wise-cracking street gangs.
Side-note: this is why I miss old-fashioned letters. Before the age of email, IM, Facebook, and so on, our expectations for instantaneous responses weren't quite so high. Plus, the USPS is an excellent scapegoat.
Short Answer Part I: WTF? Sorry, that was a short question.
Short Answer Part II: What happened? Were you too aloof? Too eager?
Short Answer, Part III: When do we stop blaming ourselves when the don't respond? Better yet, when do we stop caring?