Facebook debugger: why complaining about app_id missing

You need to add this into your head tag in HTML source

<meta property="fb:app_id" content="XXXXXXXXXXXXXXX" />

That is default app id.

Replace XXX With your app id

If you don't have an app to associate the website to, then that warning can be ignored.

For an example of a successful website which doesn't have a fb:app_id included, you can run a debug on this stackoverflow page's URL within the Facebook Object debugger, you will see that it also produces the same warning message.

The posts themselves will generally appear the same as they are represented at the bottom of the debugger tool.