ReactJS is the brainchild of Facebook and Instagram, and hence requires ample support from the development team of the two software giants. Further, it is open source, which means it thrives on contributions from the developer community worldwide. Facebook had originally created React for internal purposes, but recognizing its potential, they decided to share the project with the world. While part of Facebook uses ReactJS, all of Instagram has been built on ReactJS.
Major websites are adopting React for designing the UI component of their website, a big argument in its favor is that it has reduced bug incidents by a huge margin.