As discovered by Gizmodo's Kashmir Hill, Facebook builds a complex advertising profile for every one of its users containing everything from their two-factor authentication phone number to information gathered from other people's profiles.
Phone numbers handed over by users to Facebook for two-factor authentication are being used by the social network to add an extra targeting vector for advertisers using its platform.
This has been discovered by a research team after phone numbers they used for test accounts were being targeted by advertisers in about a couple of weeks.
As a Facebook spokesperson told Hill, "we use the information people provide to offer a better, more personalized experience on Facebook, including ads. We are clear about how we use the information we collect, including the contact information that people upload or add to their own accounts. You can manage and delete the contact information you've uploaded at any time."
Instead of using two-factor authentication phone numbers as an extra security measure, Facebook added them to their ad-targeting profiles
Moreover, she also recommended to users who are bothered about this practice to use an e-mail address for enabling two-factor authentication for their account.
The weird part in all of this is that Facebook decided to allow their users to use e-mails for two-factor authentication about four months ago, in a blog post by Scott Dickens.
"In describing this work to colleagues, many computer scientists were surprised by this and were even more surprised to learn that not only Facebook, but also Google, Pinterest, and Twitter all offer related services," Alan Mislove one of the researchers said. "Thus, we think there is a significant need to educate users about how exactly targeted advertising on such platforms works today."
The question is what will Facebook's next move be: will they choose to reveal all the information they gathered to construct ad-targeting profiles for their users or will they wait it out, hoping for the entire thing to blow over.