Facebook Inc has inadvertently exposed 6 million users' phone numbers and email addresses to unauthorized viewers over the past year, the world's largest social networking company disclosed late on Friday.
Facebook blamed the data leaks, which began in 2012, on a technical glitch in its massive archive of contact information collected from its 1.1 billion users worldwide.
As a result of the glitch, Facebook users who downloaded contact data for their list of friends obtained additional information that they were not supposed to have.
Facebook's security team was alerted to the bug last week and fixed it within 24 hours.
In a blog post Facebook said the cause of the bug was "pretty technical" but that the problem was tied to its "Download Your Information" tool.
The company uses the information that users upload to better tailor the friend suggestions it issues.
The bug caused some of this information to be inadvertently stored in association with a person's contact information as part of their Facebook account.
As a result, if someone downloaded an archive of their Facebook account through the "Download Your Information" tool, they may have been provided with additional addresses or telephone numbers for their contacts or people with whom they have some connection.
Because the contact information was provided by other people on Facebook, it was not necessarily accurate.
But Facebook did not publicly acknowledge the bug until Friday afternoon, when it published an "important message" on its blog explaining the issue.
A Facebook spokesman said the delay was due to company procedure stipulating that regulators and affected users be notified before making a public announcement.
"We currently have no evidence that this bug has been exploited maliciously and we have not received complaints from users or seen anomalous behavior on the tool or site to suggest wrongdoing," Facebook said on its blog.
While the privacy breach was limited, "it's still something we're upset and embarrassed by, and we'll work doubly hard to make sure nothing like this happens again," it added.
The breach follows recent disclosures that several consumer Internet companies turned over troves of user data to a large-scale electronic surveillance program run by US intelligence.
The companies include Facebook, Google Inc, Microsoft Corp, Apple Inc and Yahoo Inc.
The companies, led by Facebook, successfully negotiated with the US government last week to reveal the approximate number of user information requests that each company had received, including secret national security orders.
(With AP inputs)