All Facebook apps now use app-scoped identifiers. This means that the identifiers used for a particular user are unique to that application (as well as globally unique), so that a user who logs in to more than one application will be represented by more than one identifier. In order to map multiple identifiers to a single user, we have implemented Facebook’s business mapping API to get all the identities of the user. In order to take advantage of this process, you will need to set up Facebook Business Manager.
Anyone who has more than one Facebook application used for social login should use Facebook Business Manager.
Yes, we will attempt to merge your Facebook users even if you don’t have Business Manager set up. This will work in most cases, but there are a few edge cases where the merge fails, which is why we recommend the Business Manager for any customer who has more than one Facebook application.