Hackers have reportedly released data of around 81,000 Facebook accounts, and offered to sell private messages that belong to their users, reported the German news agency.
Facebook suspects that the account information offered for sale may have been gathered by "malicious browser extensions," which can range from online shopping assistants to ad blockers.
They are generally available through browser makers' stores and have access to information about people's web activity across online services they use.
Guy Rosen, Facebook's vice president of product management, said: "We have contacted browser makers to ensure that known malicious extensions are no longer available to download in their stores and to share information that could help identify additional extensions that may be related."
"We have also contacted law enforcement and have worked with local authorities to remove the website that displayed information from Facebook accounts," he added.
Many of the compromised accounts belong to Facebook users in Ukraine and Russia, but some are from the US, the UK, Brazil and other countries, the BBC reported.
The hackers claim to have details from 120 million accounts. They are offering to sell them at 10 cents per account. The BBC noted that it is unlikely Facebook would have missed such a huge breach.
However, the BBC did confirm that more than 81,000 accounts published as a sample contained private messages and that five Russian users affirmed that their messages were among them.
One of the sites where the data was published seems to have been set up in St. Petersburg, the BBC said.
The sellers told the BBC that the hacked data was not related to the Cambridge Analytica scandal or to the data breach that Facebook reported in September.
