Is it possible to use Facebook API to ID a user across apps, across businesses, for 3rd party service?

Question

We are developing a service where users upload images of themselves from which we then produce a 3D avatar looking like the user. This is quite a mathsy little problem and thus we are, alas, better at math than web development :)

When the algorithm has finished segmenting the photos and constructed 3D-model and texture we upload these to a server. We would prefer, for user simplicity, to handle identification using the Facebook API. This is implemented in our current version.

Now to the problem: When a user play a game or uses an app where they want to play as themselves they should only need to login with Facebook and we then should retrieve 3D model and texture from our server using the user ID from Facebook as identifier. But we just realized that the user ID is different for each app! In the documentation for the Facebook API we found that, for one business, you can use the business identifier across several apps, but only the apps you make yourself. And we want other developers to be able to use our avatars in their own games.

Or is there a way to use Facebook ID here that we are missing, so that we can create the avatars and other developers can make games where the users can play as themselves? Because we really want to see football games where we can play as ourselves with our friends on the team, and zombie games where the zombies are our loved ones turned undead! This last part is part of the reason, apart from simplicity, why we want to use Facebook login actually...

Answer 1

Facebook especially avoids what you are trying to do. So, even if you somehow find a workaround, there is a high possibility that it will be patched.

From your explanation, I understand that when your services used by other apps, you don't want users to give permission to your Facebook app. You want other apps to query user avatars by user's Facebook ID. Since I don't see any security concern in this logic (because anyone can access anyone's avatar just with Facebook ID), I think you are better off using email addresses and just work like Gravatar.

In the comments, it is mentioned to using other Facebook user info (name etc) to create a unique identifier. They are not guaranteed to be unique and users can easily change any info you got. So, it is not really an option.