Also here, which IMHO is more informative:

https://news.ycombinator.com/item?id=8901357

Yeah, this discussion points out an important point that it's easy to miss if you scan Facebook's patent grant: The license Facebook grants you is invalidated by any action that even alleges that any of Facebook's patents is invalid.

E.g. if you so much as Tweet that one of Facebook's patents, even one entirely unrelated to React, is invalid, you've lost your license.

Compare this with the patent grant in the Apache license, as used in a library like Angular: https://github.com/angular/angular/blob/master/LICENSE

Here the license is only terminated in the event that you institute litigation alleging patent infringement by the software itself. You're still free to challenge shitty patents and call out companies about them. Not so with Facebook's license.

This is an interesting thing (to me!).

Can you give a license that says, words to the effect, that if one accuses you of a crime then the license is revoked? This would make licenses beholden to not report the licensor for criminal activities.

IPR infringements tend to be torts, which is different, but I'm still interested in how far you can push license terms before something in the law, USC here presumably, pushes back and says "woah, you can't do that".

Anyone?

No. In context it refers to "filing any ... action." Lawsuits and similar procedures in other legal forums are often called actions. Think the movie called A Civil Action staring John Travolta about a civil lawsuit. The concern here is not how it is triggered (it must be by a lawsuit or something similar) but rather the balance of power it provides if you get into a war with Facebook, since you cannot undo having used their software.

In DannyBee's original post, he says that the key words there are "for anyone that makes any claim (including by filing any lawsuit, assertion or other action)", and that by including the words for "assertion or other action", it encompasses claims outside of a court of law. A post on Hacker News or a tweet on Twitter is an assertion, and so would fall under this. Otherwise why include that language?

IANAL, but he is, and this is his area of expertise (IIRC he runs the open-source program for Google).