@Jesse Kornblum Why is this important? Isn't the BSD license it had sufficient?

We're not updating the terms of the license. Perhaps a better title would have been, "Reassigning the copyright from employee to employer". Wesley is a Facebook employee and agreed to transfer the copyright from himself to Facebook. He still retains fully copyright to versions of the file from before he joined Facebook. (Which AFAIK, is identical to the current version). So really we're just adding a copyright notice for Facebook.

Thanks @Jesse Kornblum.