Many of the most dynamic public companies, from Alibaba to Facebook to Visa, and the most valuable start-ups, such as Airbnb and Uber, are matchmakers that connect one group of customers with another group of customers. Economists call matchmakers multisided platforms because they provide physical or virtual platforms for multiple groups to get together. Dating sites connect people with potential matches, for example, and ride-sharing apps do the same for drivers and riders. Although matchmakers have been around for millennia, they're becoming more and more popular--and profitable--due to dramatic advances in technology, and a lot of companies that have managed to crack the code of this business model have become today's power brokers. Don't let the flashy successes fool you, though. Starting a matchmaker is one of the toughest business challenges, and almost everyone who tries to build one, fails.