Why Can't I Remember?

Has someone ever introduced you to a friend, colleague, relative, or what have you, and you shake their hands, look them in the eye and exchange pleasantries, only to then promptly and completely forget their name? And I mean the instant they turn away you have no idea what their name is, and if someone put a gun to your head and demanded you say that person's name you'd be a dead man?
    Happens to me all the time. Just this evening, as a matter of fact, I was at a party where I knew exactly two (2) people. I got there kind of early - as always - so there were only a few others around. My friends introduced me, and I talked with these people, sometimes at length, about everything from football to politics to the parking ticket I was sure to get (which I did). As the crowd got larger I even introduced myself around, making small talk, getting to know people, and generally being a gracious guest.
   But I will be damned if I can tell you anyone's name. They said it when they introduced themselves, the hosts called them by name, I even used their names in conversation, trying to reinforce the association in my head. But when I try to think back and remember even the first syllable of a name, my mind is a total blank.
   Nothing.
   This is a problem. Why can't I remember these names? And it's not just with new people, it's with old people too. I'll get Facebook friend invites, and I can see that this person is 'friends' with people I'm 'friends' with, but I cannot for the life of me remember who that person might be. One of my friends from high school who also went to college with me has a freaky Rain-Man memory for all sorts of things, so I'll turn to him sometimes, or my sister has a high school yearbook from my Senior year she'll look people up in to help me out. Sometimes I'll end up remembering these people, vaguely, but most of the time I don't.
   So if you see me on the street, say 'hi' to me. I'll probably smile and wave back. But don't expect me to remember your name.