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.

For what it's worth, PHP 5.4(without APC) performed better than both Python and Ruby on that language shootout page: http://benchmarksgame.alioth.debian.org/u64/which-programs-a...

By that standard, anything may as well be written in a language that eventually results in machine code being executed.

Actually, facebook is moving to a JIT model and getting better performance out of it than the crosscompiled code.

http://arstechnica.com/business/2011/12/facebook-looks-to-fi...