Facebook is sharing personally identifying information that you never submitted to the social network with its advertisers – and you can’t erase it from their records.
That’s from a paper (PDF) published by researchers at Northeastern University and Princeton University, who looked into how ad targeting works on Facebook.
Say you’ve got a mobile phone number and a landline, and you’ve listed only the former in your profile. If an advertiser has that number in their database, they can use it to target you with ads on Facebook. But if the advertiser has the landline number, they could target you with that as well.