Already done!
More specifically, there are portions of the main Facebook app which is built in React Native, and other portions which are not (bet you can't tell), and that's likely how things will continue for the foreseeable future.
React Native can be used in an incremental way, and we definitely take advantage of that. We're building a lot of new things in React Native, and we're converting some things where it makes sense, however we never convert things simply for the sake of having done so. There's a lot of good reasons to leave some of our highly performing products as they are for now.