The social network's technology manages a vast and rapidly expanding web of connections for its millions of users.
Facebook is a wonderful example of the network effect, in which the value of a network to a user is exponentially proportional to the number of other users that network has.
Facebook's power derives from what Jeff Rothschild, its vice president of technology, calls the "social graph"--the sum of the wildly various connections between the site's users and their friends; between people and events; between events and photos; between photos and people; and between a huge number of discrete objects linked by metadata describing them and their connections.
The middle tier consists of caching servers. Even 800 database servers can't serve up all the needed data: Facebook receives 15 million requests per second for both data and connections.
Read the rest of the article (I understood about 30% of it, but it was still interesting to see the architecture)