If you are using or considering using React in a project you might want to consult a lawyer. Because of the patent clause you are not allowed to do anything that constitutes as competing with Facebook. If you do take legal actions or in other ways challenge Facebook, your license to use React is immediately revoked.
With Facebook having so many products already, and constantly acquiring more, it’d be hard to start a new product on React.js without risking that your license for it being revoked – even if they weren’t a competitor before you started. In reality I doubt it’s a problem, but what a clause!