Facebook users are reporting that their accounts have been flooded by images of hardcore pornography and violence.
In what appears to be the latest attack on the social network, some members have complained that the graphic images have been appearing in their news feeds which they have apparently 'liked', all without their knowledge.
Others are being told by their friends and contacts that they have been sending messages to click on videos and images.
The hack appears to be a typical Facebook hack in which users unwittingly give permission for an outside application to post images and videos to their account.
Facebook recently revealed that it faces up to 600,000 such attacks every day.
Users are outraged, however, and some have been threatening to close their accounts.
"Since when did Facebook become a porn site?" asked one user on Twitter. "I'm so close to closing my account."
"It's like a demon hacked into everybody's Facebook," wrote another user.
The actress Courtney Zito told The Christian Post, who broke the news of the hack: "My feed is littered with porn. I can’t even check my news feed with anyone around because of it. Just saw one with a guy who had his skull bashed in and his brains on the street … I’m about ready to deactivate…"
There is speculation that the hacker collective Anonymous might be behind the attack.
The group had threatened to "kill" Facebook on 5 November though no attack emerged.
Facebook has not yet commented on the hack.