This study examines the aggregate bandwagon effect of popularity cues on the viewership of online user-generated videos. Cognitive and behavioral theories of information processing suggest that Web users, overwhelmed by information and quality uncertainty, will gravitate toward the popular choices made by earlier decision makers, which appear via indicators such as hit counts to forge quality impressions. Building on the theories, we hypothesize that how much viewer exposure videos will attract at any future time depends on their viewership accumulated individually; furthermore, this viewership cascade is moderated by pictorial and verbal preview because such information reduces quality uncertainty for content shoppers. Our longitudinal model tests these hypotheses using an extensive real-life dataset on video clips retrieved from a video-sharing site.