Note.txt

Don't forget to add app_id at string.xml 

<?xml version="1.0" encoding="utf-8"?>
<resources>
    ...
    <string name="app_id">272943959560173</string>
</resources>

then add meta and activity at manifest

<meta-data android:value="@string/app_id" android:name="com.facebook.sdk.ApplicationId"/>
<activity android:name="com.facebook.LoginActivity"></activity>