I use React all the time for individual components on server-rendered pages. It feels like a natural extension to formerly monolithic Rails/Node/etc apps.

That said, when an SPA is called for I'd far rather use Ember and get routing, state and a hundred other things for free, without needing to bolt together an entire front-end framework from parts each time.

Of course, hence the focus on server-side rendering. It is quite difficult to do properly though. Facebook itself is not a single page application.

Yep! I find myself using React.js for complex forms/wizards on non-SPA websites far more often than using it to build SPAs.

Absolutely! I'm current using it for a entirely back-end routed Symfony app. It's perfect for any front-end stuff complex enough that jQuery would become a mess.