This seems to me like just another backdoor way of being able to build a more robust database of personal information on you. With your mobile number and the numbers of all your friends, in coordination with the cell carriers (or NSA, whichever you prefer) they can tie together data about who you call & how often with your friend activity on Facebook. Google has been doing it too, asking for a "mobile number backup" when you log into Gmail.

Just the next erosion of our privacy, disguised as a protection of our privacy.