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Have you tried disconnecting and reconnecting your Facebook Account?
To unlink StumbleUpon from your Facebook account, simply go to http://www.facebook.com/editapps.php in your Facebook account and remove the StumbleUpon app. This will unlink your StumbleUpon account from your Facebook account.
You may also go to http://www.stumbleupon.com/settings/ in your StumbleUpon account, select the "Connected Accounts" tab on the left and click on "Unlink my Facebook account."
To reconnect go to your Connections tab and click the connected your account link.
If this does not work, please write back with your username and we'll investigate.