The revelation comes courtesy of a team of computer scientists who were able to precisely target adverts at individuals using only phone numbers and email addresses which had not been explicitly shared with Facebook.
To pull off this feat the researchers took advantage of a little-known trick which sees Facebook allow advertisers to upload a database of phone numbers or email addresses for individuals they wish to target.
In this way companies can harness data to place specific ads before people on their existing mailing lists in what the social network itself describes as a ‘custom audience’.
Facebook goes one step further than this basic form of targeting however by accessing a deeper layer of data behind the top-level phone number and email address of specific individuals, by making use of contact details handed over for security purposes as well as information contained in connected individuals contact books.
The sneaky nature of this approach is compounded by Facebook’s recalcitrance in being fully transparent about the extent of these measures by both refusing to divulge the extent of ‘shadow’ information it holds and denying those who want it deleted, claiming that such data is owned by the second party who shared their address book.
Responding to Gizmodo, which first reported these criticisms, Facebook said: “People own their address books. We understand that in some cases this may mean that another person may not be able to control the contact information someone else uploads about them.”
Facebook is one of a number of US tech firms facing the prospect of an antitrust probe by the White House to counter 'platform bias' in the sector.