What decides the order of friends on facebook chat?

[The Philosopher]

If you open facebook and see the chat box on the right you can see a list of some of your friends some of which are online and some aren't (or maybe they are but have chat off?) I was wondering what factors make that specific group of people appear more than others?

I ask because I asked out a friend of mine and she said yes. We're going out next week but I've noticed shes climbed that list to the top over the last couple days and I'm wondering is she checking my profile out?
Is it profile views?
The thing is we really don't talk on Facebook itself, we mostly text or call so I'm just curious.