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It's always been written in PHP, and they released a compiler for the language that they developed in house:

http://www.insidefacebook.com/2010/02/02/facebook-open-sourc...

and MySQL http://www.facebook.com/MySQLatFacebook

http://blog.facebook.com/blog.php?post=7899307130

More interesting are how they use technologies rather than only which one, f.ex this: http://www.allfacebook.com/2010/02/facebook-makes-their-web-...

Don't forget about Cassandra, which they developed just for scalability issues that they were having, and is now used at Digg and other sites (I think reddit uses it for something as well now) http://www.facebook.com/note.php?note_id=24413138919

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