Facebook Algorithm

What was that algorithm that Mark Zuckerberg's friend gave him when creating Facebook?

It's a pretty damn simple website, what algorithm could he possibly have needed from a math major?

Well the problem lies into having only two choices,
the pics are selected randomly, so hypothetically if an ugly girl
is always in competition with some other uglier girl,
chances are she's gonna top the list.Whether a prettier girl
will rank lower.
The simpler solution is to test each contestant linear to other
but that would be O^2