Facebook has promised to address complaints it was responsible for wiping email contacts in some users' mobile phone address books, replacing them with @facebook.com listings.
The social network blamed the deletion of users' contacts on a software bug.
The problem followed last week's move to make Facebook's own addresses visible by default on its website, hiding the ones originally listed.
Facebook said it was fixing the issue, promising it "will be resolved soon".
The social network linked the issue to the application programming interface (API) at the heart of a tool designed to ensure that when its members amended their contact details, the changes would be made to their Facebook friends' smartphone address books.
The software was designed to copy over its users' "primary email addresses" - the ones they used to log into the social network.
However, a statement acknowledged that "for people on certain devices, a bug meant that the device was pulling the last email address added to the account rather than the primary address, resulting in @facebook.com addresses being pulled".
Since Facebook's synchronisation tool only synchronised email addresses that were visible on its site, and it had made third-party addresses invisible by default, pre-existing contacts were deleted as a result.
One company-watcher told the BBC the error risked angering users already annoyed by Facebook's deliberate change to contact listings on its site.
"The gravity of changing personal data on users' phones is much greater than just changing them on a cloud-based service or a website," said Anthony Mullen, senior analyst at the tech consultants Forrester.
"The lesson here is Facebook should have offered a simple wizard walking people through the change showing what impact it would have rather than just letting it happen automatically.
"However, despite talk of a backlash it doesn't seem these problems have been grave enough to have motivated users to quit the network."