How does Facebook choose the order of your top friends online?

Okay, so you know how on fb news feed page (not your profile page), they list your top 20 friends that are online right now. Do you know how they choose the order? I know that its defiantly not random bc my bff is always first, followed by my next bff, and so on. But, does it involve how often they look at your page, pics, etc etc. Thanks for the help!