> Its application code is still developed using PHP This is surprising since 1) Facebook is most likely using Hack and 2) it could save a lot of money by moving to Go. > Facebook’s entire site runs on HHVM (desktop, API and mobile) Isn't Facebook running mostly on React? I'm guessing HHVM only really powers the API, business logic, etc.
The majority (all?) of the front end and mobile apps are powered by React and React Native, respectively.