Most of Facebook is now written in Hack, an internally-developed language it introduced when its existing methodologies proved too inflexible. Facebook has now open sourced Hack so other developers can use it.
Facebook explains the need for Hack this way: "Hack reconciles the fast development cycle of PHP with the discipline provided by static typing, while adding many features commonly found in other modern programming languages." Hack code is designed to run in parallel with PHP, so you can gradually introduce Hack onto a site without having to recode everything.
Hit the link below to learn more about Hack and install it. It runs on a wide variety of Linux distributions. There's also a nifty interactive tutorial on the site.