"Because Facebook's entire code base is compiled down to a single binary executable, the company's deployment process is quite different from what you'd normally expect in a PHP environment. Rossi told me that the binary, which represents the entire Facebook application, is approximately 1.5GB in size."

http://arstechnica.com/business/2012/04/exclusive-a-behind-t...

It's not compiled ahead of time any more, but yeah, that gives you an idea of just how darn big the server side code is.