The existing literature on two-sided markets addresses participation externalities, but it has neglected pecuniary externalities between platforms. In this paper we build a model that incorporates both externalities. In our set-up, differentiated platforms compete in advertising levels and offer consumers a service free of charge that is financed through advertising. We show that advertising can exhibit the properties of a strategic substitute or complement. Surprisingly, we find that platform profits can increase with market entry and that there are cases in which the level of advertising rises with entry. We also consider endogenous entry and provide a welfare analysis.