Native iOS app: What do I put in Facebook's developer "iPhone App Store ID" under Settings, Basic?

Create a new app in iTunes Connect. That app will have a unique app id. That's the id they want. That is, in fact, the App Store id for the app.

The accepted answer is true, but the iOS SDK docs also say

iPhone App Store ID - If you have an iPhone app that is listed in the iTunes App Store enter the ID here, e.g. 123456. If Facebook links out to your app and finds that the user does not have your app installed, then it will take the user to your app in the store. The user can then install your app. If your app is not in the iTunes App Store you can enter any valid app's ID but be sure to update this when your app is approved in the App Store.

Emphasis mine.

This is what I did. I use an existing public app ID (picked one at random from the store) because my app isn't out yet..

Right now, the simplest solution is to just type in "0" as iPhone App Store ID - a solution which is also demonstrated in the Facebook SDK Getting Started Guide: https://developers.facebook.com/docs/getting-started/facebook-sdk-for-ios/3.1/