IIUC, the main thing that Google's internal codesearch does that makes it superior to external systems (outside of an IDE, like GitHub code search) is that Google actually builds everything, and so it can incorporate that information into its index. There's only so much text search can do when you have macros generating code.
Internally, we use variants of our pipeline to index a variety of open source repos, and some non-blaze/bazel internal repos. Those are often non-Google repos. But we're using some internal postprocessing and serving logic to actually create and host the final index.
Unfortunately I don't know if there's any significant use of Kythe outside of Google. We get a handful of questions on the open source repo from time to time, but that's all I know about.
