Messages from at least 81,000 Facebook accounts are reportedly being offered for sale.
Hackers are said to be offering details from 120 million accounts - many of which belong to users in Ukraine and Russia, but also includes some from the US, UK, Brazil and other countries - for 10 cents per account.
As reported by the BBC, Facebook will most likely not have missed a breach of this size, but more than 81,000 accounts published contained private messages.
Five Russian users have confirmed their correspondence is among them.
BBC added that of the sites here the sample data was published appears to have been set up in St. Petersburg.
The sellers also told the broadcaster that the hacked data was unrelated to the Cambridge Analytica scandal, or a data breach reported by Facebook in September.
The social network itself has investigated the claims, and it suspects the inforation was gathered by "malicious browser extensions".
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."