At the scale that Facebook operates, a lot of traditional approaches to serving web content break down or simply aren’t practical. The challenge for Facebook’s engineers has been to keep the site up and running smoothly in spite of handling close to half a billion active users.

Pingdom has published an article: Exploring the software behind Facebook, the world’s largest site, which takes a look at some of the software and techniques they use to accomplish that.

Source: http://royal.pingdom.com/2010/06/18/the-software-behind-facebook/