Example 3 adds a nonsense method, EmailUser#send_to_feed. What does that mean? Email users don't have feeds. If we're going to evangelize OO purity, let's do it right.

    class Post
      def created
        user.post_created(self)
      end

      def send_to_feed(feed)
        feed.send(contents)
      end
    end

    class TwitterUser
      def post_created(post)
        post.send_to_feed(twitter)
      end
    end

    class EmailUser
      def post_created(post)
        # no-op.
      end
    end

The post merely tells its user that it was created. Then the user can decide to do something else. All users know about posts, but only TwitterUser knows about feeds.

Honestly though, if I saw the not-so-good code, I'd leave it alone. It takes a pretty big justification to double the code for the same features.