The main BSD license file says plainly that Facebook grants you the right to use the software. Nowhere does it say that it is merely a grant of copyrights. While the Facebook Additional Grant of Patent Rights purports to be purely additive, its mere existence changes how I would interpret the scope of that main license file. As such, I find the meaning and implications of Facebook's contract to be unclear. I would not want to depend on it, personally.

The feeling is a bit like stepping into a restaurant that offers lead-free burgers for only $0.99 extra. Theoretically, declining the upcharge is no different than eating anywhere else. Yet...

I am not a lawyer. This is not legal advice.