I\'ve been writing a game for Facebook using Rails and jQuery. Since I started using the Facebook Javascript SDK, using localhost as an app domain seemed to work just fine.
This works for me in heroku, the localhost thing didn't work (for me). I hope it helps
1.) In the center under the first box of options, click "+ Add Platform" and choose "Website" (or whatever is appropriate for your app.)
2.) In the box that comes up for the website you just added: Site URL: http://MYAPP.herokuapp.com/ (replace MYAPP with the name of your app)
3.) In the box above that (Settings => Basic): App Domain: MYAPP.herokuapp.com (replace MYAPP with the name of your app)
4.) At the bottom right - click "Save Changes"