Facebook says a bug in its system led to users' contact information — such as email addresses or phone numbers — to be accessed by other users who either had some contact information about that person or some connection to them.
The social media company said in a blog post that the cause of the bug is "pretty technical" but that the problem is 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.
Facebook said it has fixed the problem and is in the process of notifying affected users via email.
The affected accounts represent only a fraction of the over 1 billion users on the social media site.
Facebook, which is headquartered in Menlo Park, California, said that it has no evidence that the bug has been used maliciously and it has not received complaints.