Platform businesses add value by facilitating interactions between customers who are attracted in part by network externalities. Two-sided platform businesses with low costs of reversing participation status have become more important with the rise of the Internet. This essay is concerned with new businesses of this sort and the initial critical mass hurdle that they generally seem to face. In a very general model, we show how this hurdle depends on the nature of network effects, the dynamics of customer behavior, and the distribution of customer tastes. Weak, plausible assumptions about adjustment processes imply that platforms must get a sufficient number of members of both sides on board to launch successfully.
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