I'm not sure what you mean by "large" but Jane Street has apparently millions of lines of OCaml written. Of course, I'd argue that the fact that PHP is inherently unsuitable for anything complex doesn't keep Facebook from having a gigantic amount of it. The difference is that they had to write a type checker for it :)
I think the truth is more simple. Traditional OO is what is taught, traditional OO languages have tons of libraries, and there are tons of legacy code in traditional OO. You can easily find OO programmers. Nobody ever got fired for making OO systems, even when they end in barely-maintainable horrors full of mutable state.
> I'm not sure what you mean by "large" but Jane Street has apparently millions of lines of OCaml written.
Ocaml is a nice, pragmatic hybrid of imperative, OO and FP. Adding sporadic side-effects to some component (1) will not force the rest of your program interacting with it into some monad. I guess there's a reason they didn't use Haskell :P
(1) For example, you want to compute on-line summary statistics, where the input is run-time configurable, i.e., items can come from a file, network or memory stream.
That's certainly one (optimistic) conclusion. Another could be that FP is not suitable to large, complex projects.