The World Wide Web supports new styles of creative activities. For this study, we investigate massively collaborative creation via the Web, by which numerous people gather to evolve their works collaboratively. Nico Nico Douga is a video sharing website, where many videos are created collaboratively. We specifically examine Hatsune Miku, a version of singing synthesizer application software that has inspired not only song creation but also songwriting, illustration, and video editing. As described herein, different types of creators interact mutually to create new contents though their social network. Using tags, we classified videos and creators on Nico Nico Douga automatically into four basic categories. Thereby, we produced a social network from relationships among videos and creators by analyzing videos' descriptions. The social network reveals interesting features. Different categories of creators serve different roles in evolving the network, e.g., songwriters gather more links than other categories, implying that they are triggers to network evolution. We also extracted communities from the network and observed different community structures. One is a centralized network in which a single songwriter is central and others are peripheral. The other is a messier network, in which some illustrators are central, but the centrality is weak.