Facebook is reigniting concerns by allowing marketers to target ads at its 900 million users based on the email address and phone number they list on their profiles, or based on their surfing habits on other sites.
Facebook is using data to study the links between Facebook ads and members' shopping habits at brick-and-mortar stores, as part of an effort to prove the effectiveness of its ads, reports the Wall Street Journal.
Facebook hasn't said which advertisers participate in the studies.
Gokul Rajaram, who manages Facebook's ad products, told the Wall Street Journal: "We have been working to make it easier for marketers to reach the right people at the right time and place."
Facebook says it doesn't sell data about individual users to advertisers, or even let them directly see the data.
In September, Facebook began allowing companies with their own lists of email addresses and phone numbers to target ads at specific groups of Facebook users of at least 20 at a time. If the business list of emails or phone numbers matches any emails or numbers in Facebook, then an ad appears on the user's page.
Over the summer, Facebook also began using its identity data to experiment with selling ads on other websites and apps.