"Counting by lines of code, roughly one-third of the codebase on each platform is shared C++ code."
Source: https://code.facebook.com/posts/498597036962415/under-the-ho...
They are using a custom version of Djinni for cross-platform support.
Looks like Microsoft is going down a similar path with Visual Studio:
https://www.visualstudio.com/en-us/features/cplusplus-mdd-vs...