I've done a little thinking, and I'm pretty sure I've gotten Facebook's ***ic Top Story thing figured out:
There's two ways a post can become a top story, one is that if it's marked as a top story by one of your friends, which in the case of several top stories then it goes by the number of comments and likes.
Two, if none are marked as a top story, then it goes by the number of comments and likes on the post. Or it's just plain randomness.
I think that's how it works ;)