Update to Facebook instructions?

Facebook changed the setup for their app page, so I want to make sure I have all the steps for getting the code needed. I've created an app, and know where the key is, but am unsure of where to enter the site URL, which is important. They also have a notice about upgrading API? I'd appreciate any insight. Thanks.

Also, curious if this plugin works with Disqus or if there are plans to integrate it in future. ???