After the shutdown of Parse, I don't see how anyone could avoid having doubt about Facebook's commitment to React Native, or to anything else outside of their core business.
They announced it in Jan 2016 and gave one years warning and shut it down in Jan 2017. Parse open sourced many of its components and provided an easy migration path for applications depending on it.
React Native, IMHO, is a fundamental part of their core business. Among many things, it lets them ship updates and more efficiently A/B test features more frequently. If Facebook gets sick of RN, at least it's a completely open source library that's able to be transitioned to fully community supported.
The FB app uses RN for very esoteric and small portions of the flows. It has 230MB (at least) of compiled binary.
Instagram also uses it minimally, mainly to replace what used to be webviews.
Those are not "major parts".
In the Instagram app, really only the settings screens and the bookmarks feature are written in RN - 2 of the simplest features in the entire app.
Push Notifications, Edit Profile, Photos Of view, Post Promote, Comment Moderation, Lead Gen Ads, SMS Captcha...and that was in February. They are ramping up their usage all the time.
I want to see examples of React Native usage in the feed or the main profile architecture. Something critical.
Most portions of the app is rendered using Component Kit.