I think what's really missing is some sort of PM system within the gameplay. I mean, if anyone on your friend list has their gates open, it's easy to find out. You simply tell the guard that you either want to go out, and he then looks for an open gate, or you open your OWN gate, at which time he alerts you to the fact that one of your friends has their gate open.
But, see, that's just it... someone's gate has to be OPEN in order for any of this to work. And there are so many things you can't do within the game with the gates open, that most of the time, unless you KNOW someone is coming over, you keep them closed.
Here's what Nintendo SHOULD have done. If someone on your friend list is playing animal crossing, you can tell (maybe with an icon or whatever in the friend list). Then you send them a quick message to see if they want to come over. That message could then be hand delivered in the other town by Pete the mailman. He could do the whole, running over to you with the "!" over his head, and deliver the PM.