When you setup the Facebook SSO application on https://developers.facebook.com
, the app is set as private and only the person who created it will be able to login. until you make it public. Until this is done they will get an error:
If you are ready to have this option available to your customers to login to the storefront using Facebook, you need to activate the application and remove it from development mode. In order to activate the App, go to https://developers.facebook.com
> Login > App > From the left menu select "App Review" and there you'll see an option to activate the application and set it as public. Once you do this all the users will be able to use the Facebook button on your site to login.