There are other companies (I would be betraying confidences to list who) that have looked at it and decided not to use it because of the strong retaliation clause. I should elaborate that those companies are not planning to sue FB as far as I know, but they want to preserve the opportunity to do so (i.e. they don't want to de-weaponize their patent portfolio with respect to FB).
In effect, it's more, not less, permissive and honest. INAL, but that's what I've been told by one.
To answer your original question, I found this statement by Automattic's legal counsel:
> “Automattic looked at the legal issues with Facebook’s patent license on React,” Sieminski said. “The termination provisions of the patent license aren’t ideal, but are not a blocker to using React as part of Calypso.”