if there is the default Facebook App is installed in your phone or if you are trying to upload your app on Google Play, then you need a different KeyHash. because, when u developed your FB app you have registered your FB app with a key hash which is created by using the default debug.keystore.
so, now u need a different KeyStore to generate the different KeyHash. u need a signed KeyStore, which u can get by the help of this tutorial , after generating new keystore, generate a key hash and register your FB App with this new key hash. this should work. for generating new KeyHash u can follow this link.