Is Facebook's code so monolithic that they can't deploy new, decoupled services written in new languages? Twitter did this with Ruby, Java, and Scala. Didn't Facebook create Thrift RFC for exactly this purpose??
As you say, Thrift connects this all. You can look into our C++ core library, Folly, and other C++, C, Python, and Java applications and libraries like Rocksdb, Presto, watchman, Buck, flint, scribe, and so forth on github.