This is not to diss any particular language but an architectural problem - frontend and backend are very tightly coupled which makes it difficult to develop anything outside this stack. By splitting a humongous (and always growing) homogenous application into smaller services you get smaller failure domains and better interoperability with acquisitions and potential new groundbreaking projects that do not easily fit into the current development model. Again, I have no opinion about HHVM and Hack which are completely orthogonal to the problem at hand.
The coupling has nothing to do with the language and everything to do with the fact that FB grew organically into this giant monolithic mess of spaghetti. Can you do SOA in PHP? Of course. Does FB do it? Nope. Will this bite them in the long run? I believe so.
