“They found that when a user gives Facebook a phone number for two-factor authentication or in order to receive alerts about new log-ins to a user’s account, that phone number became targetable by an advertiser within a couple of weeks. So users who want their accounts to be more secure are forced to make a privacy trade-off and allow advertisers to more easily find them on the social network.” Gizmodo: Facebook Is Giving Advertisers Access to Your Shadow Contact Information. (This is a good example of self-regulation at work.)