The existing literature on two-sided markets addresses participa-tion externalities but so far 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.

We propose both a monopoly and a duopoly model of a two-sided market. Both settings are fully comparable, as we impose a homogeneous good produced at zero costs without capacity constraints, as well as identical parameterization of market sizes. We determine the duopoly equilibrium and the monopoly optimum in terms of the parameters and obtain solutions with and without subsidization (prices below marginal cost) of one market side. We show that there exists a continuum of economically plausible parameter sets for which duopoly equilibrium prices exceed optimal monopoly prices and one with no observable price effect of competition, i.e. one where optimum and equilibrium prices become equal. Despite the fact that virtually everything except for the number of platform operators is identical in the latter situations, total demand on both market sides in the duopoly market exceeds total demand in the monopoly market. Furthermore, even though there is no observable price effect, there is still a competitive effect in so far that total profits in the duopoly equilibrium are strictly smaller than monopoly profits. The relationship of total welfare is ambiguous in subsidization cases, while it is strictly greater in duopoly, if no subsidization takes place. Our results sharply contradict economic intuition and common economic knowledge from one-sided markets.