We have just started a sample app for demonstrate building apps with ReactJS and ASP.NET Web API. An early version of the app can available on github at https://github.com/shijuvar/react-aspnet . Please note that the ReactJS version of the app is not a final version, but it is an early version which will continuously refactor with a proper Flux architecture. The RESTful API written with ASP.NET Web API 2.
We have been using MVC for building web apps and REST services for long time. At server-side, MVC is really good choice for building web apps. At one day, someone else has started to use MVC for building client-side apps too and provided a MVC framework for front-end apps. Then everyone followed that convention for solving all kinds of problems in client-side apps. IMHO, using MVC for client-side apps is really bad idea and I believe that MVC doesn't scale at larger cleint-side apps. The real beauty of ReactJS is that it is keeping away from existing conventions and patterns and trying to bring some practical solutions for solving the problem around building larger front-end apps which can be easily scale-up with recommended architecture practices such as Flux.
The source code is avaialble in github at https://github.com/shijuvar/react-aspnet