This study examines the impact of both demand and supply side factors on long tail and superstar effects in the context of online software download. Our descriptive analysis suggests the coexistence of a steeper head and a longer but slimmer tail. Employing a novel empirical approach via the quantile regression model, we find a significant interaction effect between the demand-side factor (online user reviews) and the supply-side factor (product variety) on users’ software download. The influence of the two factors and their interplay on long tail and superstar effects vary significantly across different product popularity level. The results highlight the importance of incorporating both supply and demand factors in long tail research. The findings also offer an explanation for the mixed results reported in extant studies on the influence of online user reviews.