However, Facebook uses HipHop (http://developers.facebook.com/blog/post/358) to make PHP faster (so it's not the plain-vanilla PHP).

From the linked article:

HipHop programmatically transforms your PHP source code into highly optimized C++ and then uses g++ to compile it. HipHop executes the source code in a semantically equivalent manner and sacrifices some rarely used features such as eval() in exchange for improved performance. HipHop includes a code transformer, a reimplementation of PHP's runtime system, and a rewrite of many common PHP Extensions to take advantage of these performance optimizations.