FBSDKAccessToken iOS SDK , identify a unique user for data

I am using Facebook login OAuth for my app. The reason behind posting this question is uniquely identify the Facebook id , by using userID or access token.

The data will be stored in a database, and should be uniquely identifiable for that Facebook logged-in user.

The currentAccessToken does change and we can configure a notification(FBSDKAccessTokenDidChangeNotification) when it changes.

So, I confused, that should I use userId or accessToken.

As, in case if user deletes the app. and wants the data back for the app. there should be unique identifier for that user.