Note — Sep 30, 2018

Facebook Is Giving Advertisers Your Shadow Contact Information

Even if you make efforts to protect your privacy, Facebook will still build a shadow profile—in part by cross-referencing your contact info uploaded by friends—and offer it up to advertisers as targeting options.

“I think that many users don’t fully understand how ad targeting works today: that advertisers can literally specify exactly which users should see their ads by uploading the users’ email addresses, phone numbers, names+dates of birth, etc, 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. Thus, we think there is a significant need to educate users about how exactly targeted advertising on such platforms works today.”