Whatever protocol Facebook is using internally to interoperate between their thousands of servers, with some additional authentication and authorization layers

Whatever protocol Facebook is using internally to interoperate between their thousands of servers, with some additional authentication and authorization layers put in? I mean, not sure how sensible that would really be, but I think it's worth considering that Facebook is not some PHP script with a small local MySQL database behind it either, but rather a massive distributed system in itself, even if under the administrative control of a single entity.