Facebook pretty much has to use php, they can't afford to rewrite all of this huge codebase. They have written their own compiler that translates php to c++ and then uses g++ to compile that into native code. I'm sure they had some pretty serious reasons to do that. And they probably wouldn't have to do that sort of thing if facebook wasn't written in php in the first place. Thus, the argument "hey, facebook is written in php so let's use php" is invalid, it's a typical "cum hoc ergo propter hoc" fallacy.