For projects the size of Facebook there's a lot of risk and not a lot of reward in rewriting the whole site. Instead of retraining all their developers to write

For projects the size of Facebook there's a lot of risk and not a lot of reward in rewriting the whole site. Instead of retraining all their developers to write in a new language and taking the time to stop all current development and begin the rewrite, they were able to have a few guys go off and make the current infrastructure faster. If you have a sizable existing PHP codebase time and money would be better spent making it run on HHVM than rewriting it in another language, especially if you have an existing army of PHP developers.