Quote: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."