if you make any sort of patent claim against Facebook
My understanding (and I'm asking for correction, if I'm wrong) is that last year FB changed this provision to only cover patents on technologies within React itself, not "any sort of patent claim against Facebook", which WAS the situation before they changed it.
So, the current state is a grant of "you are licensed to use our React patents at no cost as long as you don't try to make a legal claim that any of our React patents are not legally ours."
The old grant terminated if, among other things, you argued that any Facebook patent was invalid or unenforcable. This meant that if Facebook sued you over an unrelated patent, you couldn't defend yourself without the React patent grant terminating.
The new grant specifically exempts any counterclaims; you can't lose your patent grant just because you defend yourself. You can still lose it if you sue Facebook over unrelated patents.
My understanding (and I'm asking for correction, if I'm wrong) is that last year FB changed this provision to only cover patents on technologies within React itself, not "any sort of patent claim against Facebook", which WAS the situation before they changed it.
So, the current state is a grant of "you are licensed to use our React patents at no cost as long as you don't try to make a legal claim that any of our React patents are not legally ours."