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True, they use hacklang but it's no longer folowing the php spec https://hhvm.com/blog/2019/02/11/hhvm-4.0.0.html

Does anyone know what language(s) they tend to use for more "backend" style services?

Mostly Hack and C++ as far as I can tell from glancing at their recent ACM paper: https://dl.acm.org/citation.cfm?id=3338112

Nice, a very fresh source (August 2019)!

"C++ is the main language of choice for backend services. There are 10s of millions of lines each of mobile and backend code."