Facebook has landed itself in hot water with its users for a second week in a row, after a bug caused those using Facebook's sync tools to lose contacts from their smartphone address books. This follows the social networking giant's decision last week to make their email addresses more prominent, even going as far as to make existing users' non-Facebook emails invisible on their profiles and replacing them with @facebook.com addresses.

Unfortunately for Facebook, the changes appear to have introduced a bug which caused the primary email addresses of contacts synchronized via Facebook to be deleted from the contact list when using certain mobile platforms. It's believed to affects phone users who have activated the "contact sync" feature on their BlackBerry and Android smartphones, as well as those using beta versions of Microsoft Windows Phone 8 and Apple's iOS6.

According to Andrew Bosworth, Facebook's director of engineering who spoke with The Verge, the bug caused contact synchronization to pull the last updated email address for contacts only – which happened to be the @facebook.com email addresses, and in the process removed the original ones from handsets.

Contact synchronization on devices is performed through an API. For most devices, we've verified that the API is working correctly and pulling the primary email address associated with the users' Facebook account. However, 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 email address, resulting in @facebook.com addresses being pulled. We are in the process of fixing this issue and it will be resolved soon. After that, those specific devices should pull the correct addresses.

Bosworth says the bug will be resolved by the end of tomorrow, if not earlier and would then synchronize each users primary email address as well. However, he also pointed out that users will need to change their email accounts' visibility settings if they want their original email address to be visible on their Facebook profile pages.

Users can do so by clicking on Update Info on their Timeline and then editing the Contact Info box.