Ok, so expanding on that, the problem becomes this:

1) You create a successful app using React

2) Facebook blatantly copies your app.

3) You can't sue Facebook

Is such a clause actually enforceable? Has this been tested in a court of law? Just curious.

Well, the clause isn't "you can't sue", it's "you have to stop using React if you sue".

But what if you don't stop using it?

You can be sued by Facebook for infringing their license.

Which brings us back to my original question ;)