Step 3: Obtain a Facebook App ID

If you haven't already registered your app with Facebook by creating a Facebook app, you should create a new app:

 
And fill in the information:

Once your app is created, or if you already had a Facebook app, the app ID is shown at the top of the dashboard page.

If you already have a Facebook app (and corresponding Facebook app ID) that you're usingfor the same app in another platform (for example, Android), you can use the same app ID for iOS.

Next, you need to set the Bundle Identifier and configure your Facebook application to support login from an iOS application.

Select "Settings" from the left-hand nav. Inside the settings, click on "Add Platform" and choose "iOS". Then provide your Bundle Identifier in the "Bundle ID" field and enable "Single Sign On". Don't forget to save the changes!

You can find or set your Bundle Identifier in Xcode in the Project tab.