If you have an active site and then decide to enable any social authentication provider (Facebook, Twitter, etc) there is no "merge step" for current users where DNN recognizes that the user is already a member of the site and then wants to authenticate via a social authentication provider. We need to implement a step where DNN says "Hey, you're already a member of this site and now you want to login with Facebook" and then merges the user's social profile credentials with their current DNN site profile credentials.
As it currently stands a user would not be able to authenticate via the social authentication providers if they are already members of the site. As such, it only makes sense to enable social authentication providers on brand new sites. Adding in the "Merge Profile" step would allow site admins to ensure a smooth authentication process for their end users regardless of when they activated the social authentication providers.