The Facebook app is not written in React Native. They tried it and famously abandoned it no?
The Ads Manager app is written in React Native.
The Instagram app is written in React Native.
The entire FB app is not written in React Native yet, but overtime I'd expect more things to be.
The statement that FB "abandoned react native on mobile" is demonstrably false.
Someone currently on the Instagram team can correct me if I'm wrong.
Second, they serve different purposes. Litho will be used for FB scale optimization of certain intensive views. React Native is used for fast developer productivity. I'd wager that RN will be used in most places and Litho is used in hot spots sparingly.