I had this problem too, here is how i solved it.
Check if your site is using www or not, so it will be either http://www.yoursite.com
OR
http://yoursite.com
Go to your facebook application and make sure the “connect url” on the connect tab is exactly the same as how you see your site url when looking at your site. So if your site doesnt use www make sure you dont have www in your facebook connect url.
Should solve the problem.
