Just curious: did Facebook change the license? AFAIK they can revoke permission to use from 3rd parties. Am I mistaken? If not, isn't this a huge risk for startups?
It's purely defensive, they'll only revoke your license if you sue them for patent infringement. And they only revoke the patent-grant of React as far as I can tell, not your license to use it (and there aren't any patents related to React as far as the internet claims).
Which in turn means they can infringe on any of your patents with no consequences. If they do, your only choice is to rewrite your entire application in a hurry, and only then you can sue them.
It's purely defensive, they'll only revoke your license if you sue them for patent infringement. And they only revoke the patent-grant of React as far as I can tell, not your license to use it (and there aren't any patents related to React as far as the internet claims).