> Today, our Android apps for Facebook, Messenger, and Instagram each have more than 1 million lines of Kotlin code, and the rate of conversion is increasing. In total, our Android codebase has more than 10 millions lines of Kotlin code.

Probably 1000 engineers working on an app with each writing 1000 lines of code. I am not surprised Facebook app has >1 million lines of code. The app is bloated with features that it's user unfriendly.