Here’s a number to start your day: the folks who run Airbnb believe they will earn as much as $3.5 billion a year by 2020, before interest, taxes and depreciation, according to Fortune’s Leigh Gallagher. That’s an audacious goal—up 3,400% from last year’s earnings, and more money than made by 85% of the Fortune 500. It would propel CEO Brian Chesky and his partners into the pantheon of business success, and be yet another demonstration of the incredible power of digital platforms.

It would also be yet another demonstration of why the economics of digital businesses are driving increased inequality in the modern world. Consider this: Since its founding nine years ago, Airbnb has spent less than $300 million of the $3 billion it has raise from outside investors since it was founded. It’s not an apples to apples comparison, but Hilton had $7.2 billion in expenses in the first nine months of last year alone. To be sure, Airbnb helps people make a living by renting out their property. But when it comes to jobs, its contribution in comparison to the hotels it competes with is minuscule.

You can read the remarkable saga of Airbnb in Gallagher’s new book: The Airbnb Story: How Three Ordinary Guys Disrupted an Industry, Made Billions…and Created Plenty of Controversy. Or you can read this excerpt that ran in Fortune. I highly recommend both.

But when you are done, consider the fact that we still haven’t figured out how an economy increasingly characterized by such zero-marginal-cost businesses really works, and how it distributes technology’s benefits fairly to society. That’s the challenge for the future.

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Update: An earlier version of this story said that Airbnb has spend less than $300 million on all of its expenses since the company was founded. In fact, Airbnb has spent just $300 million of the $3 billion it has raised from outside investors.