This usually means that you setup your Facebook Application
incorrectly. It's easy to fix though.
First, go here: https://developers.facebook.com/apps
There you will find your Facebook Application. Click the "Edit
Settings" link on the Settings area.
The basic underlying problem here is that you have the wrong values on
this page. The "Site URL" needs to be the *exact URL of your site*.
And this does mean *exact*. Whatever shows up in the Browser URL bar
is what you *MUST* put here. It's amazing how many people think their
website has a "www" in it when it doesn't, for example. Or how people
just use capital letters wherever the heck they please. These things
*DO* matter, the values must be EXACTLY correct.
If you own your own domain name, then you can optionally fill it in on
the site domain field as well. However, this is not required.
Fixing that should generally eliminate your problem.