I'm surprised Facebook doesn't use more Go. Everyone I talk to from Facebook seems to love Go--no idea why they don't use it more widely. Facebook doesn't strike me as the sort of company to not use a programming language just because its rival (Google) is in the driver's seat. Go is still open source.

I know Facebook has some D advocates, but D doesn't seem to be too popular among the programmers I know. I haven't looked too deeply at it myself, but it seems to be a language more interested in taking the job of Rust and C, rather than the job of Python and Ruby. So there is no reason Facebook can't pursue both languages as a company.