You might want to uninstall the Facebook app: There are reports that Facebook’s iPhone app is infiltrating the address book that comes preinstalled on the iPhone, erasing contacts’ email addresses, and replacing them with their Facebook address (you know, the [name]@facebook.com email address that everyone’s given and nobody uses). This is bad enough, but to make matters worse, the @facebook.com addresses—which are supposed to redirect to users’ Facebook inboxes—apparently aren’t working all that great. The end result is that emails sent by many well-intentioned iPhone users are disappearing into the abyss, irretrievable by their intended recipients. Presumably, affected users are also losing the real email addresses of their friends. This is, to put it lightly, completely inexcusable, and it doesn’t take too much thinking to conjure up many, many disastrous scenarios that could result from a reckless policy like this. source
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I never had the app on my phone in the first place and this is an exact scenario where you should not trust FB in the future. Delete it NOW.