Thanks for the source and the reply. It's good to know that in 2014 they had promos in developed city markets that dropped their gross margins into negative territory.
Nonetheless, my impression is that this is more the exception than the rule, based on the financials I linked to earlier.
From their 2015 financials, it looks like revenues have been consistently higher than cost of sales.
Source: http://www.nakedcapitalism.com/2016/11/can-uber-ever-deliver...