Is Facebook iOS app built in React Native?

It's also easy to build part of your app in React Native, and part of your app using native code directly - that's how the Facebook app works.

This is the official words from Facebook. it means Facebook did not use completely react native. it is partially using react native in their apps.

Facebook claims:

"Using React Native in the Facebook app".

It means that they have used React Native partially in app. But the point is to what extent they have used React native, no one knows.

So, it seems like most of the core is build in native but they have started implementing new modules in React Native.

One of my friend who was interviewed at FB 5 months ago was told that more than 80% of the application is still native.

I realized what is the point of moving over to React Native from Native for big companies? rather stay native and give the best user experience!