I crawled the MIT facebook and built a friendship predictor that will take guesses at who, out of the people you are not currently listed as being friends with, you actually are friends with in real life. People's traits (age, sex, home state, etc.) are significantly less predictive than how close you are on the [ friendship / class / photo / group ] network. In descending order of predictive power, the most important traits when considering two people as potential friends are:

how many friends you have in common how many pictures you simultaneously appear in how many classes you have together relationship status looking for (e.g. friendship, dating, random play) how many facebook groups you have in common