Facebook has said millions of Instagram users were impacted by an internal security lapse which made their passwords available for employees to search.
When the breach came to light four weeks ago, it said "tens of thousands" had been affected but now says the number is vastly higher.
The social media giant said in late March it had inadvertently stored passwords in plain text.
This made it possible for its thousands of employees to search them.
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It said the passwords were stored on internal company servers and no outsiders could access them.
It also said in March the issue affected "hundreds of millions" of Facebook Lite users and millions of Facebook users.