What kind of programming language does Facebook uses

Answer 1

good question, facebook use many technologies. as far as I know, they used PHP until they created their own language, Hack, based on PHP. they also created their own engine (HipHop) to compute PHP and Hack.
they also use heavy client side code with JavaScript librairies (at least jQuery and react, probably some more).
you can read more about facebook technologies from their blog : https://code.facebook.com/posts/

Answer 2

php