[chrisco255]

Microsoft builds Skype on React Native. They use React in production on several websites including outlook.com. They are using it internally and not just for external devs.

Also see this HN thread from a year ago: https://news.ycombinator.com/item?id=12108273

[nojvek]

You'd be actually surprised how many teams inside Microsoft wrote their own smaller versions of OS libraries. It happened a lot less after Nadella era, but Ballmer era had a "we can't use anything not built here"