Personal information relating to around six million people have been accidentally exposed by a bug in Facebook's data archive.
Telephone numbers and email addresses were shared with people who would not usually have access to the data.
As yet there is no suggestion that there was a sinister reason for the information getting out and Facebook said it was "upset and embarrassed" by the error, which was discovered by a programmer outside the business.
The firm added that the bug occurred due to the way it handled contact lists and address books uploaded to the social network.
Usually it analysed contact details and names in order to make friend recommendations and put people in touch with those they knew.
However, the glitch led to some of the information Facebook generated during that checking process being stored alongside the uploaded address books and contact lists.
So when users downloaded their profile the extra data travelled with it, allowing others to view contact details which had not been specifically shared with them.
The bug has now been fixed, said Facebook.
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