Step 1). Plug your phone on your pc and run the following command on your terminal:
adb logcat > c:\logcat.txt
Step 2). Then, run the app and try to log in on facebook.
Step 3). Go back to the terminal and quit the logcat.
Step 4). Open up your file created with notepad and press “control + F” find for the term “hash” and voi-la. You may find your missing hash, just write down the value and paste it into your facebook´s app configuration page in the Hash place. Enjoy!