We study a dynamic duopoly model with network externalities. The value of the product depends on the current and past network size. We compare the market outcome to a planner. With equal quality products, the market outcome may result in too little standardization (i.e. too many products active in the long run) but never too much. The potential inefficiency is non-monotonic in the strength of the network effect, being most likely for intermediate levels. When products differ in quality, an inferior product may dominate even when the planner would choose otherwise, but only if the discount factor is sufficiently large