code: def get_backend(self, backend_name):
          
          if backend_name == 'twitter':
              from social_friends_finder.backends.twitter_backend import TwitterFriendsProvider
              friends_provider = TwitterFriendsProvider()
          elif backend_name == 'facebook':
              from social_friends_finder.backends.facebook_backend import FacebookFriendsProvider
              friends_provider = FacebookFriendsProvider()
          elif backend_name == 'vkontakte-oauth2':
              from social_friends_finder.backends.vkontakte_backend import VKontakteFriendsProvider
              friends_provider = VKontakteFriendsProvider()
          else:
              raise NotImplementedError("provider: %s is not implemented")
  
          return friends_provider
docs: returns the given backend instance