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When people create a local account with user name, email, password and etc., they can log in to the Azure AD B2C Portal via username@tenant.onmicrosoft.com.
Then is it possible to let them log in to the portal via social identity provided by Facebook or other external services? I don't feel it's viable though.
No it is not. Azure Portal uses "regular" Azure AD as its identity provider. Thus it will not redirect the user to login with their identity provider. You can only login using accounts that exist in an Azure Active Directory.