Facebook Callback or Cross-Domain Receiver

Does it work for you in both configurations, and always not just him? It should be whatever page you're visiting from (with or without the www). So for our demo page, we use a callback of:
<!-- m --><a class="postlink" href="facebook.sourcecoast.com/">facebook.sourcecoast.com/

If you can give more information about the times it works (with you) and times it doesn't (with or without you), we should be able to get to the bottom of this.

This is really an issue of how you get to the website. For both of the sites below, if you go to 'k9athlete.com/' or 'columbus-salsero.com/' the login box works. If you put the www. at the beginning of either though, it will fail. This is because Facebook requires that the site's URL be the exact same as the callback URL in the application. The only way to fix this is to setup a redirect in your htaccess file (or your SEF component of choice) to always redirect users to the correct address... so if they go to www.k9athlete.com">www.k9athlete.com it will redirect them to the 'correct' k9athlete.com">k9athlete.com site.