The reason Facebook hasn't migrated away from PHP is because it has incumbent inertia (it's what's there) and Facebook's engineers have managed to work around many of its flaws through a combination of patches at all levels of the stack and excellent internal discipline via code convention and style - the worst attributes of the language are avoided and coding style is rigidly enforced through a fairly tight culture of code review (failing to adhere to the style and "going cowboy" by writing sloppy code results in pitiless mockery by one's peers).  Engineering management has never had to ...