On pingdom.com there's a recent article posted looking at some of the technology behind Facebook and how they manage to keep up with the huge demand put on them and their servers.

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. This article takes a look at some of the software and techniques they use to accomplish that.

They have to be able to handle over 570 billion page views a month across over 30,000 servers so there's several key technologies than they use to help make this a reality. Besides the famous HipHop for PHP tool, they also use things like memcached, Cassandra, Hadoop and Varnish. There's also other non-tech specific practices they do to keep things flowing smoothly.