Facebook reported a bug in its system on Friday, saying users' information may have been exposed to other users who had "some contact information about that person or some connection to them."
The social media website, which has since repaired the glitch, said about six million Facebook users had their email addresses and telephone numbers shared.
The issue was caused by a problem with the "Download Your Information" tool.
"If a person went to download an archive of their Facebook account through our Download Your Information (DYI) tool, they may have been provided with additional email addresses or telephone numbers for their contacts or people with whom they have some connection. This contact information was provided by other people on Facebook and was not necessarily accurate, but was inadvertently included with the contacts of the person using the DYI tool."
All users that were affected by the glitch have been contacted via email, Facebook said, adding that it has as yet received no complains. "In almost all cases," Facebook said, information was supplied to only one person, and was not accessible to advertisers.