To add to what others have said most Facebook hires learn PHP during the boot camp process. The continued use of the PHP is definitely not due to lack of experience with other languages or some misplaced fondness for the language.
When you work on a codebase the size of Facebook's the core language becomes less important than the abstractions and framework that have been built around it. As well as having sane internal libraries Facebook have added extensions to the language like XHP and the yield keyword (http://www.serversidemagazine.com/news/10-questions-with-fac...) which make it much nicer to use.
I should amend my previous comment though; most place I've worked at that are within a couple of orders of magnitude of Facebook's size have developed or adopted abstractions that constitute the majority of code engineers interact with.