Photo: GETTY IMAGES
Hackers appear to have compromised and published private messages from at least 81,000 Facebook users' accounts.
The perpetrators told the BBC Russian Service that they had details from a total of 120 million accounts, which they were attempting to sell, although there are reasons to be sceptical about that figure.
Facebook said its security had not been compromised.
And the data had probably been obtained through malicious browser extensions.
Facebook added it had taken steps to prevent further accounts being affected.
The BBC understands many of the users whose details have been compromised are based in Ukraine and Russia. However, some are from the UK, US, Brazil and elsewhere.
The hackers offered to sell access for 10 cents (8p) per account. However, their advert has since been taken offline.
"We have contacted browser-makers to ensure that known malicious extensions are no longer available to download in their stores," said Facebook executive Guy Rosen.
"We have also contacted law enforcement and have worked with local authorities to remove the website that displayed information from Facebook accounts."
The breach first came to light in September, when a post from a user nicknamed FBSaler appeared on an English-language internet forum.
"We sell personal information of Facebook users. Our database includes 120 million accounts," the user wrote.
The cyber-security company Digital Shadows examined the claim on behalf of the BBC and confirmed that more than 81,000 of the profiles posted online as a sample contained private messages.
Data from a further 176,000 accounts was also made available, although some of the information - including email addresses and phone numbers - could have been scraped from members who had not hidden it.
The BBC Russian Service contacted five Russian Facebook users whose private messages had been uploaded and confirmed the posts were theirs.
One example included photographs of a recent holiday, another was a chat about a recent Depeche Mode concert, and a third included complaints about a son-in-law.
here was also an intimate correspondence between two lovers.
One of the websites where the data had been published appeared to have been set up in St Petersburg.
Its IP address has also been flagged by the Cybercrime Tracker service. It says the address had been used to spread the LokiBot Trojan, which allows attackers to gain access to user passwords.