They mention this in the Facebook Effect (http://www.amazon.com/Facebook-Effect-Inside-Company-Connect...). Or, at least the part about an subdomain for testing their website in-house. Everything else was me analyzing that.
We also maintain a robust employee list that is cached in APC on every web host that you can always call an is_employee style function for any user ID on. The careers site in particular has some employee only functionality that this endpoint is probably checking.
I have no doubt that their internal employees or a subset of them are using a different build of Facebook, and maybe I'm missing something, but I don't understand how this is related.