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"…
Bullshit conspiracy theories. How can something that is opt-in be an erosion of privacy? By default it is not enabled. Just don't use it and you're fine.
Hyperbole would be a more apt description.
How can something that is opt-in be an erosion of privacy?
Quite easily. You can choose to use a service without fully understanding the privacy implications. I don't think we can expect the general public to be infosec and personal rights experts.