Facebook’s data breach was smaller than first feared, the network has now said, but for those most seriously affected losses are vast and highly personal, BBC repoirts.
Its estimate has been revised down from 50 million users to 29 million.
Facebook said the FBI had requested it did not discuss who may be responsible for the attack, which was first revealed last week.
The breach centred on the use of access tokens, which allow users to log on more easily.
Users can visit this link to find out if they have been directly affected.
"We have not ruled out the possibility of smaller-scale attacks, which we’re continuing to investigate,” Facebook’s head of product management, Guy Rosen, said in a blog post.
"Of the 50 million people whose access tokens we believed were affected, about 30 million actually had their tokens stolen,” he said.