The harvesting occurred when users were offered email password verification as an option to verify their identity when signing up to Facebook, a method widely condemned by security experts. In some cases if users did enter their password, a pop-up message would appear informing them that it was "importing" their contacts, without even asking their permission to do so.
These contacts were then fed into Facebook's database systems and used to build a map of users' social links and inform recommended friends on the social network. It's not clear if the data was also used for ad-targeting purposes.
In a statement given to Business Insider, the company said that these email contacts had been "unintentionally uploaded" to Facebook when users created their account.
It also said that prior to May 2016, it offered an option to verify a user's account and voluntarily upload their contacts at the same time. However, the feature was changed and the text informing users that their contacts would be uploaded was deleted, but the underlying functionality was not. Facebook says at no point did it access the content of users' emails.
We estimate that up to 1.5 million people's email contacts may have been uploaded. These contacts were not shared with anyone and we're deleting them. We've fixed the underlying issue and are notifying people whose contacts were imported. People can also review and manage the contacts they share with Facebook in their settings.The news is just the latest addition to a long list of privacy blunders and violations by Facebook. In March, for example, it emerged that between 200 and 600 million Facebook users may have had their account passwords stored in plain text in a database accessible to 20,000 Facebook employees. Some Instagram passwords were also included.
That was followed earlier this month by news that cybersecurity researchers had discovered millions of Facebook records publicly accessible on Amazon's cloud servers, after the data was uploaded by third-party companies that work with Facebook.
In yet another development just this week, over 4,000 pages of documents from 2011 to 2015 were leaked which provide insight into how Facebook took advantage of user data while publicly promising to protect user privacy before and after its 2015 move to end broad access to user data.