The Facebook mobile application sure is large, but I wager lots of that is assets and repeated code. Even so, there is lots of code there anyway (someone dove into that at some point), but I think that's a "different kind" of complexity than, say, a compiler. There's lots of complex user interfaces to set up and work with, and there's some amount of user state to keep track of; but a compiler has to perform highly complicated operations on a large, intricate state that can often be described effectively with a strong type system.

I'm inclined to believe a compiler is better suited for a type system like Haskell's than e.g. a mobile app or a game engine is, but maybe someone with more domain knowledge might correct me here.