"Facebook uses hphpi (and now hhvm) for day-to-day software development, but uses the HipHop compiler (hphpc) to create optimized binaries that serve the Facebook website. hphpc is in essence a traditional static compiler that converts PHP→AST→C++→x64."
According to wikipedia: http://en.wikipedia.org/wiki/HipHop_for_PHP
"in Q1 2013, the production version of facebook.com started running on HHVM, replacing HPHPc."