FB generates a different user id for each FB application id.
"Facebook issues app-scoped user IDs for people who first log into an instance of an app, and page-scoped user IDs for people who first use a Messenger bot. By definition, this means the ID for the same person may be different between these apps and bots."
"Facebook issues app-scoped user IDs for people who first log into an instance of an app, and page-scoped user IDs for people who first use a Messenger bot. By definition, this means the ID for the same person may be different between these apps and bots."
https://developers.facebook.com/docs/apps/for-business
So the 10 billion number the researchers quoted does not necessarily represent individual users.