"MARRIAGES (could be) broken," friendships ended, and business deals compromised as part of Facebook's latest security scandal that exposed the private messages and photos of up to 90 million Facebook users.

Security experts warned the hackers could use the stolen information to blackmail the multibillion-dollar social network and, if unsuccessful, would likely "dump the data" on internet forums for anyone to read, in an act that could resemble the destructive Ashley Madison adultery scandal.

Facebook is at the centre of another security scandal.
Facebook is at the centre of another security scandal.

The warning came in addition to revelations third-party accounts using Facebook, such as Airbnb and Instagram, may also have been compromised in the breach, and as the social network faced fines of up to $US1.63 billion under strict new European Union privacy laws.

Facebook confirmed more than 50 million users had their accounts compromised by unknown hackers on Saturday, though an additional 40 million users may be affected.

Hackers gained access to Facebook using three security flaws linked to its "View As" feature, which were added to the platform in July last year.

Facebook product management vice-president Guy Rosen said it was not known for how long hackers had accessed the data or what information they had stolen.

"Our investigation is still very early so we don't yet know exactly the scope of the misuse and how and if accounts were actually misused," he said.

Exposing users’ private messages could be most damning.
Exposing users’ private messages could be most damning.

But Bitdefender senior security analyst Bogdan Botezatu said the hackers could easily have gone unnoticed in Facebook's system for months, downloading each user's personal details, photographs, likes, even deleted posts, and "everything the victims have ever posted on Facebook".

He said exposing users' private messages could be most damning.

"It could have the same impact as the (adultery site) Ashley Madison hack," he said. "This exploit could expose everything from extramarital affairs to intellectual property and other things people do not want public.

"If, at any point, they become public following a data dump, marriages will get broken, friendship will end abruptly, and sensitive pictures will flood the internet."

Mr Botezatu said the massive Facebook hack was a sophisticated attack that could be designed to blackmail Facebook or influence elections.

The data breach could also have significant financial consequences for Facebook as new European Union privacy laws could deliver fines of up to $1.63 billion if it determines the social network did not do enough to protect users' information.

The breach follows Facebook's biggest privacy scandal to date in March, when it was discovered political data firm Cambridge Analytica bought access to the private details of 87 million Facebook users and allegedly used them to influence the 2016 US election.

Sophos principal research scientist Chester Wisniewski said Facebook's new privacy failure should serve as a warning to users.

"The truly concerned should use this as a reminder and an opportunity to review all of their security and privacy settings on Facebook and all other social media platforms they share personal information with," he said.