You seem to think that because Facebook's website uses React it's a React app. Facebook is no different from other continuously developed applications that evolve organically. In other words: it's probably not a good example for a "best of breed" pure React app.

I'm guessing Facebook uses React more on facebook.com than it uses React Native in their mobile app (I think there was an article a while back on HN about the ridiculous amount of packages in their mobile app, which is a testament to their policy of bolting on new code instead of rewriting the old unless actually necessary) but it's a safe guess to say that there's more to facebook.com's frontend than just React.