The monopoly benchmark on two-sided markets

The literature on the effects of market concentration in platform industries or two-sided markets often compares the competitive outcome against a benchmark. This benchmark is either the “joint management” solution in which one decision maker runs all platforms or a “pure” monopoly with just one platform. Literature has not generally discussed, which benchmark is the appropriate one. We show that the appropriate benchmark, i.e. how many platforms the monopolist will operate, depends on whether agents multi- or singlehome, whether the externalities are positive or negative, and in some cases on the properties of the demand functions. Different situations require different benchmarks. Our results also help to anticipate the effects of proposed platform mergers, where the assessment might crucially depend on the number of platforms after a merger.