According to Facebook, the glitch was due to a bug in its application-programming interface, and makes the last added email address be pulled and added to the user's phone Contacts.
The company says they are working hard at fixing the problem, but in the meantime, a lot of users have effectively lost some of the information stored on their devices, reports The Register.
In the majority of cases, those who allowed their Blackberry, Android, iOS6 beta and Windows Phone 8 beta phones to sync their contacts with Facebook, have had the originally stored email addresses overwritten. The lucky ones had their contacts duplicated - with the new ones containing the @facebook email addresses.
The worst part of it is that many users won't discover for a while that they are automatically sending emails to the new addresses. If the recipients don't check their Facebook email addresses - and the great majority of them don't - they will be none the wiser that they received any emails.
Those that do use it might still miss them.
"By default, messages from friends or friends of friends go into your Inbox. Everything else goes to your other folder," explained a Facebook spokesperson to Mashable. "That is likely where the messages are being sent from other people’s emails. Even if that person is friends with them on Facebook, if the friend doesn’t have that email on their Facebook account, the message could end up in the other folder."
In the end, Facebook admitted that it should have notified the users about the email switch and explained the move, but denied it was taken because Facebook wanted users to switch to using the @facebook email address.
I doubt that this will mean anything to those users who have now lost a lot of their contact information, but I also doubt that this will make them leave Facebook in a huff.
I am sorry - though not surprised - to see that Facebook continues with its effective modus operandi of asking forgiveness instead of permission.
By subscribing to our early morning news update, you will receive a daily digest of the latest security news published on Help Net Security.
With over 500 issues so far, reading our newsletter every Monday morning will keep you up-to-date with security risks out there.