The Web technology enables numerous people to collaborate in creation. We designate it as massively collaborative creation via the Web. It is becoming an important activity such as Wikipedia and Yahoo! QA. As an example of massively collaborative creation, we particularly examine video development on Nico Nico Douga, which is a video sharing website that is popular in Japan. We specifically examine videos on Hatsune Miku, a version of a singing synthesizer application software that has inspired not only song creation but also songwriting, illustration, and video editing. As described herein, creators of interact to create new contents though their social network. We analyzed the process of developing thousands of videos based on creators' social networks. The social network reveals interesting features. Different categories of creators serve different roles in evolving the network. We also extracted communities from the network and observed different community structures and investigated the evolving nature of the network using motif analysis.