So, basically, if you sue Facebook on grounds of patent/copyright infringement for any reason, you don't get to use their software?

No, the license terminates when you make a claim (of which a lawsuit is a subset of) that any 'right' of any patent claim that Facebook makes is invalid or unenforceable. This would include statements to the effect:

"That Facebook patent is invalid." or "Software patents are invalid."

That plus you accept that all of Facebook's patents are valid.

No, it doesn't mean that.

It means that you lose the additional license to use any of Facebook's patents that may cover React. These may or may not exist.

You keep making definitive statements. Are you saying that you can keep using React Native if your patent rights are waived? That'd only be the case of those patents don't exist - but even then Facebook is stockpiling patents related to everything connected with social networks.

Also, they'll likely be patenting whatever they can out of this technology, whether it's valid or not, you'd have to fight any action that Facebook holds against you - in the meantime invalidating everything, allowing Facebook to sue you for violating potentially dozens of patents; most everyone can't afford a lawsuit.

Facebook open sourcing all of these things is purely a power play, and not trying to contribute to the developer community. The power that this is going to allow Facebook to leverage in the future could get really nasty - it's why Google isn't using any of it.

Then why say "whether or not such claim is related to the Software?" That would imply just the opposite to me.

That clause relates to the patents you own that you're suing Facebook over, not the patents Facebook owns that they're (no longer) granting you a license to.

As an example, say Facebook owns patent 1234567 "Method and system for reacting natively". Google owns 7654321 "High-density computers in a data center". Google builds the new Maps app using React Native. This would infringe 1234567, except that React Native comes with a patent grant, so the Maps app is properly licensed.

Then Google sues Facebook over Open Compute Project, claiming infringement of patent 7654321. This has nothing to do with React Native, but Google has just lost its license to 1234567. So Facebook now gets to sue Google over the Maps app. (More likely, Facebook will threaten to sue in order to force a settlement for the other lawsuit.)

Well, OK, fair, but for practical purposes it seems like the same thing.

Yes, and the license could easily be construed to imply that your license is immediately revoked if you tweet "Facebook infringes on patents".