The patent grant is solely a grant from Facebook to the world, so Facebook can choose to grant more rights at any time. Just like I can sign a contract with yo

The patent grant is solely a grant from Facebook to the world, so Facebook can choose to grant more rights at any time. Just like I can sign a contract with you allowing you to borrow my bicycle on Thursdays and then later unilaterally decide that you can borrow it on Mondays as well. The University of California did something like this in 1999 when they cancelled the advertising clause of the original BSD license. See ftp://ftp.cs.berkeley.edu/pub/4bsd/README.Impt.License.Change.