gistfile1.rb

# Intended Behavior:
ExternalContactList.type_to_class('facebook') # => ExternalContactList::Facebook
UserIdentifier.type_to_class('facebook')      # => UserIdentifier::Facebook
UserIdentifier::Facebook.type_to_class('facebook') # => UserIdentifier::Facebook

# Actual Behavior
ExternalContactList.type_to_class('facebook') # => UserIdentifier::Facebook
UserIdentifier.type_to_class('facebook')      # => UserIdentifier::Facebook

mixin.rb

module HasProviderSubclass
  module ClassMethods
    def type_to_class_string type
      @@type_to_class_string[type]
    end

    def type_to_class type
      @@type_to_class[type]
    end

    private

    def __setup_provider_variables
      @@type_to_class_string = Hash.new { |h, type| h[type] = "#{self.to_s}::#{type.capitalize}" }
      @@type_to_class = Hash.new { |h, cs| h[cs] = type_to_class_string(cs).constantize }
    end
  end

  module InstanceMethods

  end

  def self.included(receiver)
    receiver.extend         ClassMethods
    receiver.send :include, InstanceMethods

    receiver.send :__setup_provider_variables
  end
end

user_identifier.rb

class UserIdentifier
  include HasProviderSubclass
  class Facebook < UserIdentifier
  end
end

class ExternalContactList
  include HasProviderSubclass
  class Facebook < ExternalContactList
  end
end