Facebook doesn't just disperse your personal information to its robust network of advertisers to hawk products and services online. According to a report by Gizmodo, it also serves ads to everyone in your mobile address book, provided they use the social network.

Reporter Kashmir Hill delves deep into what she calls "shadow contact information," or the company's practice of targeting your mobile phone contacts with ads. Per Hill's reporting, which consulted a paper by professors from Northeastern University and Princeton, shadow contact information works like this:

The researchers also found that if User A, whom we’ll call Anna, shares her contacts with Facebook, including a previously unknown phone number for User B, whom we’ll call Ben, advertisers will be able to target Ben with an ad using that phone number, which I call “shadow contact information,” about a month later. Ben can’t access his shadow contact information, because that would violate Anna’s privacy, according to Facebook, so he can’t see it or delete it, and he can’t keep advertisers from using it either.

You might not think this applies to you, but it can happen in any number of ways, notably through the "Find My Friends" feature that Facebook introduced in 2014. The feature parses a user's mobile phone book to make connections and build out a profile. Those phone numbers don't remain idle data on a server—they're distributed to ad partners who then target your contacts.

Facebook makes a case for transparency in its Ad Preferences, which shows you the number of advertisers that have been supplied your credentials. My page, for example, contains a glut of advertisers that I've never supplied personal info to. Facebook says all "info was collected by the advertiser, typically after you shared your email address with them or another business they've partnered with."

Ironically, securing your account with two-factor identification might make you more visible to hawk-eyed advertisers. On a macro level, the report authored by Giridhari Venkatadri, Piotr Sapiezynski, and Alan Mislove breaks grim news:

Phone number sand email addresses added as profile attributes, those provided for security purposes such as two-factor authentication, those provided to the Facebook Messenger app for the purpose of messaging, and those included in friends’ uploaded contact databases are all used by Facebook to allow advertisers to target users.

Facebook shared several statements with Hill, always stopping short of an outright denial of her reporting and the researchers' findings.

“We outline the information we receive and use for ads in our data policy, and give people control over their ads experience including custom audiences, via their ad preferences,” For more information about how to manage your preferences and the type of data we use to show people ads see this post,” a spokesperson said.

Source: Gizmodo

[Related: New Facebook Data Breach Affects 50 Million Users]

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