If you're going to use React, you may as well use it for the entire interface. I would imagine that the only reason you get sites which use React for only part of the page, is because they're transitioning towards using it for everything.

Not at all. I've just started a new Rails project and widgets that would normally be written with a smattering of JavaScript are now self-contained React components. I can't imagine this app ever transitioning to a single-page JS thing.

I didn't particularly mean a single-page JS app. I meant, that everything on any particular page would be constructed using React components. Rather than a combination of HTML and React components.

Isn't this a good thing? It seems encouraging that Facebook use it for pretty much everything - if they keep open sourcing their work on it we'll keep having a really well proven library. Plus it forces them to make thoughtful decisions on things like backward compatibility.

Really? For me, only some parts of the page are teal/blue. On Instagram, everything is (when logged in). On Facebook, only comments on posts and other UI items related to other people (chat list and some bar on the left).