With more than 500 million active users, Facebook is the busiest site on the Internet and has built an extensive infrastructure to support this rapid growth. The social networking site was launched in February 2004, initially out of Facebook founder Mark Zuckerberg’s dorm room at Harvard University and using a single server. The company’s web servers and storage units are now housed in data centers around the country.
Each data center houses thousands of computer servers, which are networked together and linked to the outside world through fiber optic cables. Every time you share information on Facebook, the servers in these data centers receive the information and distribute it to your network of friends.
The FAQ is 3 pages and has a lot information for the data center curious.