Edit: I'm wrong. https://twitter.com/SlexAxton/status/562318547674619904
The license granted hereunder will terminate, automatically and without notice,
for anyone that makes any claim (including by filing any lawsuit, assertion or
other action) alleging (a) direct, indirect, or contributory infringement or
inducement to infringe any patent: (i) by Facebook or any of its subsidiaries or
affiliates, whether or not such claim is related to the Software,
Contrary to the stereotypes about "viral" licensing, the GPLv3's patent grant seems to be very narrowly tailored. It says that if you redistribute GPL'd code while "knowingly relying" on patent rights that you own or were granted, then you must arrange to grant those specific patent rights to the recipients. It's equitable, in the sense that the rights you have to forego are the same ones that were granted to you: only those that would necessarily be infringed by using and distributing the software.
Facebook's terms are similar to the extent that they cover patent claims "related to the Software." But the real difference (to my layman's eyes) is in the application to other patents. You're not allowed to enforce any patent claims against Facebook or its affiliates, whether or not those patents have anything to do with React. And in the reverse direction, if Facebook accuses you of infringing a patent of any kind, you have to forfeit the right to challenge the patent's validity.
That's taking a relationship that's already heavily skewed towards big companies with huge patent arsenals, and making it even more lopsided.
The license granted hereunder will terminate, automatically and without notice, for anyone that makes any claim (including by filing any lawsuit, assertion or other action) alleging ... (b) that any right in any patent claim of Facebook is invalid or unenforceable.
Doesn't that mean that if Facebook sues you for patent infringement in something else you did (possibly unrelated to React) and you disagree, they can take away your licence to use React? In other words, Facebook could arbitrarily shut down your use of React by filing a spurious patent lawsuit against you. And if your use of React was critical to your business and Facebook didn't like the look of your business, that could be dangerous.
But there is an asymmetry here. Facebook offers the open source code license and you use it fairly and per the terms. Facebook then hypothetically steals your company's patented Crown Jewels. When you send them a C&D they retaliate by revoking your license. So now they've kicked your ass twice and you've done nothing to warrant it. That would not be what I call "fair".
This entire thing is an anti-patent troll measure.