Facebook is the second most-trafficked PHP site in the world, and one of the largest MySQL installations anywhere, running thousands of databases.

(Outdated – April 2008 for illustration only)

Company’s hardware infrastructure has over 10,000 machines,
including about 1,800 MySQL database servers.
Interesting enough, that the MySQL servers is managed by only
two system administrators
Apache 1.3.41 (as webserver)
The social networking service now spreads across three data centers (Ashburn – Virginia and San Francisco, Santa Clara – California) and counts no less than 10,000 servers (69 million users).

(Outdated – April 2008 for illustration only)

Today hosting the profiles of more than 300 million users.
and using 30,000 servers supporting its operations.