FIVE STARS! This is an absolute necessity for anybody using Facebook integration. I think judging an add-on as five 'Excellent' should be based on whether or not it should be a default XenForo feature, which this definitely should.
A great additional security step if your community allows social registrations. Considering the amount of malware that is able to create social media accounts and propagate, this is a necessary security step that should be implemented into the core of Xenforo.