If you login to Facebook, go to the search bar and type in the letter A, you will see a list of your friends whose name (first/middle/last) starts with an A, but it's sorted based on who you are most likely to be looking for. I don't know exactly how they determime this since it could be based on so many factors like messaging, profile viewing, searching, photo tagging, shared events... even pokes!

It's spooky accurate and yet Facebook goes out of their way to never mention this kind of thing: they don't want us to realize how well they know us!