Facebook users are inadvertently providing access to their names and in some cases even their friend's names to advertising and Internet tracking companies, through some popular applications, the Wall Street Journal said.
According to the Journal's investigation, the issue affects tens of millions of Facebook app users, including people who set their profiles to Facebook's strictest privacy settings, the paper said.
The practice violates Facebook's rules and raises questions about its ability to keep identifiable information about its users' activities secure, the paper said.
On Sunday, a Facebook spokesman told the Journal that it is taking steps to "dramatically limit" the exposure of users' personal information.
"A Facebook user ID may be inadvertently shared by a user's Internet browser or by an application," the spokesman told the paper.
Knowledge of an ID "does not permit access to anyone's private information on Facebook," he said, adding that the company would introduce new technology to contain the problem identified by the Journal.
Facebook could not immediately be reached for comment by Reuters outside regular U.S. business hours.
(Reporting by Anand Basu in Bangalore; Editing by Louise Heavens)