I have gone through these links
Login failed invalid key error with Facebook SDK
and
http://sean.lyn.ch/2011/07/android-the-facebook-sdk-sso-and-you/
but i could not understand what hash key exactly is??? where is that used in my application?? and in the tutorials he says we need to add this key hash to the Facebook application...in my case i have a Facebook web application, is that enough or should i definitely create an application for mobile on Facebook???