How do Facebook and Google+ do their recommendations?

In making recommendations, social network recommendations take into account many factors, including how much commonality there is between potential friends, etc. A detailed breakdown of some of Facebook's Friend Rank parameters is here However, the basis of virtually all recommendations is the notion of triadic closure, where it is observed that two people with a common friend are more likely to be friends (or potential friends). We study this notion in detail, and develop friend recommendation algorithms in NETS 150, Market and Social Systems on the Internet and in a social networks project in NETS 212, Scalable and Cloud Computing.