I've added a comment box to my site. I'm using an app_id. I've added my domain to "App Domain" in the settings section for the app. I save it. I then come back and the "App Domain" has disappeared. It's like it's not being saved.
What am I doing wrong?
I found how to do this - the UI and error messages are simply terrible. First, you need to set your site url in the website section - to eg http://www.example.com . Click Save
Then you can go to the app domain field, type 'example.com' and press enter - it should highlight. You can do the same to add more domains.
This managed to get them to stick for me.
you must fill Site URL first, after that you can fill the App Domain Sorry if my english is suck
My first guess is that Facebook is buggy today (cough).
Another thing to check is the Roles section to ensure your current FB user is in the Administrators list. You should be, of course, but I know folks who have different users for different purposes and they get mixed up as to which one they're logged in as. Just an idea.
Edit: Also, on the Advanced settings page you can control your App Type. Is it still set to Web? Since the domain is less relevant for Native/Desktop, I could see them ignoring your input if it's configured that way.
来源：https://stackoverflow.com/questions/7616965/facebook-app-domain-disappears