How to properly organize the system of accounts using the possibility of logging in through social networks. networks?

Question

Social media

A user in a mobile application / site can register using:
1) Create an account on the site / application
2) Register using an existing social account. networks (Vkontakte, Facebook...)
- How to properly organize the system of accounts?
How exactly to use the SDK is understandable.
But what to do when the client no longer uses the social account. networks, but continues to use the application? - How, in this case, should he ideally be authorized?