This study takes a longitudinal perspective towards the early stage of social-media-based cultural diffusion. Based on the case of Korean artist Psy’s viral video Gangnam Style, the study reveals the temporal changes of YouTube user characteristics, user-to-user networks, the semantics and sentiment in user-generated comments. Specifically, it shows that YouTube commenters were mostly young and male, with U.S. based commenters dominating the early stage of diffusion. Later, there was an increased share of participation from non-U.S. commenters. Commenting behavior was intensive in early stages, but the interest wore off as time went by. User-to-user interactions based on comment-replying became gradually compartmentalized; in addition, the video and its artist and underlying cultural phenomenon were being evaluated against other popular culture figures and shows. The semantic analysis also shows that commenters were interested in the national cultural origin of the video.