[sokoloff]

The only thing of significant value that they give away is $20 credits to new riders and some new driver bonuses.

The number of full-fare (no subsidy to either rider or driver) Uber trips surely absorbs these incentives, leaving a positive gross margin. (There's also a miniscule computing, bandwidth, and payment processing cost to a marginal ride, marginal being defined as the nominal X+1th ride after all the costs for X rides are already paid for.)

Perhaps absolute certainty is a slight overstatement, but here's one where I'd bet eating my hat on it. I edited to insert an "almost".

[rahimnathwani]

Here in China Uber has various subsidy programmes for drivers and customers. These eat into margins and make them negative for some (but probably not all) trips.

Example: a few weeks ago, I paid ~3USD for a 13km ride that took 90 minutes (due to Beijing traffic). The driver was definitely paid more than that for the ride.

[dllthomas]

In new markets, I have seen Uber guaranteeing "if you work this much of these hours, you will make at least $X or we will make up the difference."