Right now, Uber keeps 22 cents on each dollar paid by passengers, as its fee for creating the app, keeping it working, etc. That's cheap relative to the iTunes store, which keeps 30%. It's preposterous compared to the 3% that real-estate agents get for buying or selling a home.

We really don't know what the "fair" rate is for running a ride-hailing business. We know what's been collected to date in a venture-funded duopoly with limited public disclosure.

But my guess is that 22% is not the long-term equilibrium. Lots of outside pressures (regulators; new competitors) could drive Uber's share down. It's hard to see any forces that would cause it to expand.