5 min read

Facebook is constantly under the spotlight these days when it comes to controversies regarding user’s data and privacy. A new research paper published by the Princeton University researchers states that Facebook shares the contact information you handed over for security purposes, with their advertisers. This study was first brought to light by a Gizmodo writer, Kashmir Hill.

“Facebook is not content to use the contact information you willingly put into your Facebook profile for advertising. It is also using contact information you handed over for security purposes and contact information you didn’t hand over at all, but that was collected from other people’s contact books, a hidden layer of details Facebook has about you that I’ve come to call “shadow contact information”, writes Hill.

Recently, Facebook introduced a new feature called custom audiences. Unlike traditional audiences, the advertiser is allowed to target specific users. To do so, the advertiser uploads user’s PII (personally identifiable information) to Facebook. After the uploading is done, Facebook then matches the given PII against platform users. Facebook then develops an audience that comprises the matched users and allows the advertiser to further track the specific audience.

Essentially with Facebook, the holy grail of marketing, which is targeting an audience of one, is practically possible; nevermind whether that audience wanted it or not.


In today’s world, different social media platforms frequently collect various kinds of personally identifying information (PII), including phone numbers, email addresses, names and dates of birth. Majority of this PII often represent extremely accurate, unique, and verified user data. Because of this, these services have the incentive to exploit and use this personal information for other purposes. One such scenario includes providing advertisers with more accurate audience targeting.

The paper titled ‘Investigating sources of PII used in Facebook’s targeted advertising’ is written by Giridhari Venkatadri, Elena Lucherini, Piotr Sapiezynski, and Alan Mislove.

“In this paper, we focus on Facebook and investigate the sources of PII used for its PII-based targeted advertising feature. We develop a novel technique that uses Facebook’s advertiser interface to check whether a given piece of PII can be used to target some Facebook user and use this technique to study how Facebook’s advertising service obtains users’ PII,” reads the paper.

The researchers developed a novel methodology, which involved studying how Facebook obtains the PII to provide custom audiences to advertisers.

“We test whether PII that Facebook obtains through a variety of methods (e.g., directly from the user, from two-factor authentication services, etc.) is used for targeted advertising, whether any such use is clearly disclosed to users, and whether controls are provided to users to help them limit such use,” reads the paper.

The paper uses size estimates to study what sources of PII are used for PII-based targeted advertising. Researchers used this methodology to investigate which range of sources of PII was actually used by Facebook for its PII-based targeted advertising platform. They also examined what information gets disclosed to users and what control users have over PII.

What sources of PII are actually being used by Facebook?

Researchers found out that Facebook allows its users to add contact information (email addresses and phone numbers) on their profiles. While any arbitrary email address or phone number can be added, it is not displayed to other users unless verified (through a confirmation email or confirmation SMS message, respectively). This is the most direct and explicit way of providing PII to advertisers.

Researchers then further moved on to examine whether PII provided by users for security purposes such as two-factor authentication (2FA) or login alerts are being used for targeted advertising. They added and verified a phone number for 2FA to one of the authors’ accounts. The added phone number became targetable after 22 days. This proved that a phone number provided for 2FA was indeed used for PII-based advertising, despite having set the privacy controls to the choice.

What control do users have over PII?

Facebook allows users the liberty of choosing who can see each PII listed on their profiles, the current list of possible general settings being: Public, Friends, Only Me.  

Users can also restrict the set of users who can search for them using their email address or their phone number. Users are provided with the following options: Everyone, Friends of Friends, and Friends.

Facebook provides users a list of advertisers who have included them in a custom audience using their contact information. Users can opt out of receiving ads from individual advertisers listed here. But, information about what PII is used by advertisers is not disclosed.

What information about how Facebook uses PII gets disclosed to the users?

On adding mobile phone numbers directly to one’s Facebook profile, no information about the uses of that number is directly disclosed to them. This Information is only disclosed to users when adding a number from the Facebook website. As per the research results, there’s very little disclosure to users, often in the form of generic statements that do not refer to the uses of the particular PII being collected or that it may be used to allow advertisers to target users.

“Our paper highlights the need to further study the sources of PII used for advertising, and shows that more disclosure and transparency needs to be provided to the user,” says the researchers in the paper.

For more information, check out the official research paper.

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