I'm including the official Facebook Page Plugin (not my own app) on my website. The plugin shows as a widget with a "share" button at the top. On Chrome, Firefox, iOS 8.4 Safari, clicking that goes to a share page. But on iOS 9.2 Safari (on an iPhone 6), clicking that share button shows a page with the error:
App not setup. This app is still in development mode, and you don't have access to it. Switch to a registered test user or ask an app admin for permissions.
To eliminate any other code, I made a flat HTML page that has nothing but the Facebook code and it still does this.
What could cause this?
For some reason only on iOS 9 Safari (not on iOS 8, or in Android or Desktop browsers) Facebook's Page Plugin will not work without a public app id. On all other browsers/platforms, you can copy the code directly from their Social Plugins site (via "Get Code" button) and it will work.
But in iOS 9 Safari, Facebook's share button in the iframe works only if you create an App, click yes to "Do you want to make this app and all its live features available to the general public?" under app settings and then pass that App Id to the JavaScript source call.
This might happen because in iOS 9 because it attempts to open in the Facebook app instead of the browser. This crosses over into the "app environment" and thus changes Facebook's requirements despite them kicking you back to safari with an error.