Hi all - had the same problem as reported by several others, then I found this :
Go to the app you want for user login on the Facebook Developers page
Click on the settings tab
Click add platform
Select Website
After selection it will ask for some details such as URL for your website which uses login with facebook feature, fill the form and submit it
That's all and you are done. Make sure that the app's URL is the same as the place where you're logging in from.
and it worked !!!