The problem is the startup I work for relies on patents for a big part. And if we use any Facebook stuff, which I actually wanted to do as the main developer (react and react-native), they can use our stuff without caring about our patents! Because we can't sue them!
So basically, with that license Facebook on the one hand protects itself against patent trolls. Good, fine.
On the other hand though they grant themselves the right to use any patent of any company that uses their products, because you can't sue them any more! No? That's how I read it after having read a lot of discussions, none of which talk about that major issue, because for most people in this discussion defending their own patent(s) against Facebook is not an issue, so the discussion is very heavily one-sided towards people who don't have that problem.