Competition in two-sided markets

Many markets involve two groups of agents who interact via platforms, where one group's benefit from joining a platform depends on the size of the other group that joins the platform. I present three models of such markets: a monopoly platform; a model of competing platforms where agents join a single platform; and a model of competitive bottlenecks where one group joins all platforms. The determinants of equilibrium prices are (i) the magnitude of the cross-group externalities, (ii) whether fees are levied on a lump-sum or per-transaction basis, and (iii) whether agents join one platform or several platforms.