This paper analyzes market diffusion in the presence of oligopolistic interaction among firms. Market demand is positively related to past market size because of consumer learning, networks, and bandwagon effects. Firms enter the market freely in each period with fixed costs and compete in quantities. We demonstrate that the nature of the inefficiency under free entry can change as the market grows, and more importantly, that S-shaped diffusion can be a signal that the number of firms under free entry is initially insufficient, but eventually excessive. J. Japanese mt. Economies 29 (2013) 98-116. Faculty of Economics, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-Ku, Kyoto-City, Kyoto 603-8555, Japan; Graduate School of Economics, Osaka University, 1-7 Machikaneyama, Toyonaka, Osaka 560-0043, Japan; Department of Economics, The George Washington University, 2115 G Street, NW Monroe Hall 340, Washington, DC 20052, USA. (C) 2013 Elsevier Inc. All rights reserved.