If these projects are covered by Facebook patents, it doesn't matter if their license has a patent grant since they don't own rights to the Facebook patents.

Exactly! I have yet to see a solid walkthrough of how my situation is made worse because of an explicit patent grant.

I believe the fear of a grant revocation also revoking the BSD copyright license has been thoroughly settled. If that were the case that would be an issue but FB themselves have put in writing that that is not the case.

That's never been tested in court, and requires some rather large leaps of thought.

IANAL. There's case law in copyright law concerning implicit grants. We can assume that it's at least partially applicable to patents (See here: https://en.wikipedia.org/wiki/Implied_license).