How do Facebook and Google earn money from open source projects?

When a larger community of developers are interested in your framework, you have a bigger set of testers who are going to use that framework and give feedback. More brains working on a framework, more Ideas on improving the framework and make it better. Essentially you are getting free work from people who are interested in your framework. Moreover its an easiest way of branding for most of the companies.

When Facebook launched “React” a javascript framework for web and app development, it created a stir among javascript developers, it was an instant hit around the world. Later on, several libraries related to react has been launched by the developers and start-ups around the globe. One such library is “NativeBase” contributed by GeekyAnts. It is a free and open source framework that enables developers to build high-quality mobile apps using React Native iOS and Android apps with a fusion of ES6. This open source contribution from our organization has given us applaud from all across the react community and has contributed significantly to our companies branding.