Facebook is a big user of PHP, but they don't use quite the same PHP as the rest of us. HipHop is their own open source runtime for PHP. Last week they officially launched the HipHop virtual machine, (hhvm) which will make HipHop PHP execution faster than ever.
Read the details here: http://www.internetnews.com/blog/skerner/facebook-accelerates-php-with-hiphp-virtual-machine.html