A concept of the MotionMedia contents sharing platform is proposed to realize the ubiquitous avatars society in the near future. We define MotionMedia, the 5th medium following audio, text, image, and video. MotionMedia is a physical communication medium that includes the actual motion data or force sense data of avatars. MotionMedia should be spread worldwidely as a valuable medium. To share MotionMedia contents, especially motion data, a motion data modeling and formulation system based on eXtensible Markup Language (MotionXML) is developed. MotionXML leads to specifications to convey and reproduce robot motion via networks.