yeah, to say Facebook is written in PHP is misleading. Facebook was originally written in PHP. Now it runs on the HHVM, which foregoes supporting the dangerous

yeah, to say Facebook is written in PHP is misleading. Facebook was originally written in PHP. Now it runs on the HHVM, which foregoes supporting the dangerous parts of the PHP stdlib, and offers better performance (fixing the two biggest problems with PHP). As I understand, much of their new code is written in Hack which interoperates with their legacy PHP code but supports gradual typing and other safety features (and some niceties). Facebook is written in PHP, but they have invested a lot into making their PHP not be PHP.