Here are the details provided by Facebook:
Describing what caused the bug can get pretty technical, but we want to explain how it happened. When people upload their contact lists or address books to Facebook, we try to match that data with the contact information of other people on Facebook in order to generate friend recommendations. For example, we don’t want to recommend that people invite contacts to join Facebook if those contacts are already on Facebook; instead, we want to recommend that they invite those contacts to be their friends on Facebook.
Because of the bug, some of the information used to make friend recommendations and reduce the number of invitations we send was inadvertently stored in association with people’s contact information as part of their account on Facebook. As a result, 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.
Awkward timing, considering Facebook is one of several tech companies accused of participating in the sharing of personal information with the U.S. government after former NSA contractor Edward Snowden leaked information on the NSA/Prism surveillance scandal. The post goes on to say that around six million users were affected by the bug, and their e-mail or telephone numbers were shared as a result. But so far, Facebook says there is "no evidence that this bug has been exploited maliciously."
Photo: Courtesy of Facebook Security.