The book Accidental Billionaires: The Founding of Facebook, a Tale of Sex, Money, Genius, and Betrayal by Ben Mezrich describes this (it's the book that eventually became the movie The Social Network). Basically, Eduardo Saverin and Mark Zuckerberg just wanted to get laid, so they crowdsourced a hotness database of female students.

(It's a terrible book, I don't recommend it, I DNFed it.)