First, Facebook grants you a patent license only in conjunction with the software. You are not granted a license to use a patent that React requires in some completely different piece of software. The only thing this patent grant prevents Facebook from doing defensively is countersuing the users of software that Facebook gave them for using the software that Facebook gave them. Unless that is Facebook's idea of defense, the patent grant (without the retaliation clause) does not impair the defensive capabilities of Facebook's patent portfolio in any way.
Second, the patent grant terminates if you (first) assert any patent claims against Facebook whatsoever. So in exchange for a very narrow grant of a handful of patents, you in effect agree not to assert any patent claims against Facebook for any reason, even if they're totally unrelated to the software that Facebook's patent grant came with.
IMO this sort of asymmetric grant of rights is a very bad development for open source. It's also quite disconcerting that Facebook (and it's surroundings, I realize this guy is no longer employed there) never acknowledges this even though they're surely aware of this. If Facebook were content with merely not increasing their exposure to patent problems through open source the Apache style patent grant would have sufficed. Instead they have decided to militarize their open source projects in the service of decreasing their patent exposure in completely unrelated areas.