You're confusing the Hack language (the "subset" of PHP Facebook uses -- actually a superset, as all PHP is valid Hack) with HHVM, which is a PHP runtime engine

You're confusing the Hack language (the "subset" of PHP Facebook uses -- actually a superset, as all PHP is valid Hack) with HHVM, which is a PHP runtime engine. Wikipedia recently transitioned their PHP website from running on the Zend engine to running on HHVM. It's still regular PHP code, though.