React is worked on full-time by Facebook's product infrastructure and Instagram's user interface engineering teams. They're often around and available for questions.
For longer-form conversations about React, we've set up a discussion forum at discuss.reactjs.org. This forum is a great place for discussion about best practices and application architecture as well as the future of React. If you have an answerable code-level question, please post it to Stack Overflow instead.
In the forum there's also a category for job posts and a category for discussion of our weekly meeting notes.
If you need an answer right away, check out the Reactiflux Discord community. There are usually a number of React experts there who can help out or point you to somewhere you might want to look.
Many developers also hang out in #reactjs on Freenode.