Hey, I'm an engineer on the HipHop team, and a minor-league Haskell enthusiast. A couple clarifications:
1. HipHop is itself written in C++. While this did not logically need to be the case, it is the case.
2. These days our JIT compiler, the HipHop VM (hhvm) is out-performing the PHP-to-C++ ahead-of-time compiler. HHVM is under the HipHop umbrella, and so is also written in C++. If you're vaguely aware that Facebook has some sort of high-performance PHP execution engine, but have avoided investigation because compiling PHP sounds like a drag, check hhvm out; it is fast, and you use it the same way you use a PHP interpreter.
3. HipHop is not our entire PHP stack, however, and functional languages have a significant beachhead in static analysis. pfff (https://github.com/facebook/pfff) is a sophisticated multi-language analysis and transformation framework written in OCaml. We use it for many, many purposes; e.g., at code review submission time, pfff symbolically executes the changed code to look for various dangerous idioms and highlight them as areas for the reviewer to focus attention.
4. There are other projects written in Haskell that, alas, we're not ready to talk intelligently about publicly. Watch this space, however.
Overall, FP has a sizable presence at Facebook. Haskell's presence is a bit smaller than OCaml, though that might be changing. Facebook is definitely "using Haskell more these days," especially since Bryan O'Sullivan (author of "Real World Haskell") came aboard. I expect Simon Marlow will continue to influence things in that direction. From 100k feet, Facebook's adoption of Thrift, and Thrift's tradition of supporting polyglot languages, means that a lot of "niche" languages can flourish within the larger production ecosystem.
