We study the nature of peer effects in the market for new cell phones. Our analysis builds on de-identified data from Facebook that combine information on social networks with information on users’ cell phone models. To identify peer effects, we use variation in friends’ new phone acquisitions resulting from random phone losses and carrier-specific contract terms. A new phone purchase by a friend has a large positive and long-term effect on an individual’s own demand for phones of the same brand, most of which is concentrated on the particular model purchased by the friend. We provide evidence that social learning is a central mechanism behind the observed peer effects. While peer effects increase the overall demand for cell phones, a friend’s purchase of a new phone of a particular brand can reduce individuals’ own demand for phones from competing brands—in particular those running on a different operating system. We discuss the implications of these findings for the nature of firm competition. We also find that stronger peer effects are exerted by more price-sensitive individuals. This positive correlation suggests that the elasticity of aggregate demand is substantially larger than the elasticity of individual demand. Through this channel, peer effects can reduce firms’ markups and, in many models, can contribute to relatively higher consumer surplus and more efficient resource allocation.