Customers who pay little or nothing and are subsidized by another set of customers are essential to a vast array of businesses, including shopping malls, real estate brokerages, information technology providers, auction houses, print and online media, and employment and dating services. According to one estimate, this business model accounts for a majority of the revenues of 60 of the world’s 100 largest companies.1 With the explosion in the number of free services offered on the internet, the prevalence of so-called two-sided markets is likely to grow.

A version of this article appeared in the November 2008 issue of Harvard Business Review.