plz help
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This is because the APK signature has changed, login services like Google or Facebook use the developer's signature to decide whether or not a particular app may have access to login services.
If you want more information about APK file signatures, go to: Sign your app | Android Developers.
See more information about the Facebook login SDK: Android Login | Facebook for Developers.
Follow this tutorial to try to solve your problem: How to make unsigned APK and Google+ / Facebook login | AndroidRepublic.org.
Hope this helps.
Cheers,
SnwMds.