That is, on its face, absolutely untrue. Nowhere in the React patent terms does it specify that you grant Facebook rights to use your patents--not even to ones related to React.
That's something that isn't in the React terms, nor does it seem to show up in any legal analyses published online from a lawyer making that claim. (None of the analyses I've seen, at least; if you found some, please post them.) In fact, the only place where this claim does crop up is in off-the-cuff message board comments.
FWIW, I don't use React, I don't want to, I'm not a Facebook employee, and in fact I think the world would be a lot better off with Facebook having less influence than they do today. But that doesn't change how weird it is to keep seeing comments like yours that make claims like the one above.
