The report added that the Facebook specifically uses the user's phone number, which is provided for a two-factor authentication (2FA) and a security technique that provides a second layer of authentication to help keep accounts secure.

"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," the Facebook spokesperson said.

Facebook confirmed the use of users' phone number for targeting ads after Gizmodo reported research work carried out by academics at two US universities who found that the social network uses pieces of personal information that individuals did not explicitly provide it to target them with ads.

Besides using the contact information users willingly provide to Facebook, it is also using contact information they handed over for security purposes and contact information they did not hand over at all, but that was collected from other people's contact books, the Gizmodo report said.

The study also suggests that Facebook uses  "shadow contact information" to make money via advertising.