Well…… not only but also 😀
HipHop for PHP is a source code transformer for PHP script code and was created to save server resources. HipHop transforms PHP source code into optimized C++. After doing this, it uses g++ to compile it to machine code.
Firebug addon to Firefox makes it very visual what is what and what Facebook uses for doing what. Dissecting the code-mess.
WOW! Proving that php and apache can handle massively-large scale applications.
But not only read here: http://www.makeuseof.com/tag/facebook-work-nuts-bolts-technology-explained/
The development of facebook brought forth open source technologies: https://developers.facebook.com/opensource/