I'm not sure exactly what you mean. If your site's address is http://www.site.com, in the Web Site area of your application, you should have the following settings:
Site URL: http://www.site.com/
Domain URL: site.com
If you're asking about other features or usages of your site in Facebook, let me know, and I can help you with that as well.
If you have specific examples of things you're looking for, let us know. However, here's a few examples of how to get your site more noticed in Facebook:
1) Use the Comment and Like button on your pages. When a user likes or comments on a page, that page will show up in their Facebook Wall, showing your site off to their friends.
2) Use the JFBCFan module if you have a Facebook Fan Page. This let's user's easily see, and like, that page which is good for building a reputation on Facebook itself.
3) Use the ability to post to a user's wall on registration and/or login to put a greeting on their wall when they come to your site.
For the upcoming 4.0 release, we have a few new ways to get your site onto Facebook, and especially in your Facebook Page. That release is coming out in July, and we'll have more information about the features soon.
Hope that helps.
If you want to display your site in Facebook, you can simply set the Canvas URL in your application configuration. Once you do, the page you can view your site, in Facebook show up in the Developer area which shows all the information about your application. Generally, if you name your Canvas URL "example", the URL would be:
In the 4.0 release, we'll have options to set the specific template that should be used from your Joomla site when displaying it in a Canvas view.
Hope that helps,