Facebook _doesn't_ run PHP. A large portion of it is coded in PHP, but it runs C++ through HipHop[1]. Facebook may as well be written in anything that compiles down to C++.
By that standard, anything may as well be written in a language that eventually results in machine code being executed.
Actually, the difference between compiled C++ code, and the newest version of PHP with APC/Xcache is not that drastic, but facebook needs to squeeze out every bit of performance so they can to cut down on the number of servers.