Primarily MySQL but they use several. Gotta use the best tool for the job. There's a discussion on stackoverflow about it but I can't post the link.
NOSQL like mongodb(a kind of NOSQLdatabase) etc are used primarily in facebook and other big companies like google,amazon,digg ,etc
i can say preferring one database technology suites the most with more databases types complexity increases simply
He uses multilple and heavy databases machines server. but it can be better tells by Google he must knows.
thats good if so but i really dont think so mysql is so powefull that facebook implements it .It may be some nosql type database
Facebook primarily uses MySQL for structured data storage. In fact, their team has submitted numerous high-performance enhancements to the MySQL core and Innodb plug-in. For a few means of document storage and write-heavy applications (such as inbox search), Cassandra is used. In fact, it isn't anywhere close to being the primary database platform; it used for very specific scenarios where the NoSQL paradigm fits best.
It's irrelevant what FaceBook uses. However, if you are looking for a database solution odds are mySQL is WAY MORE than enough for you or ANY company/website.