A study conducted by the MIT Center for Energy and Environmental Policy Research [mit.edu] analysed revenue and costs for over 1100 Lyft and Uber drivers, with the conclusion that most earn below minimum wage for their state and about 30% actually lose money when all the costs of owning and operating their vehicles are taken into account.
"A Median driver generates $0.59 per mile of driving, and incurs costs of $0.30 per mile", "On an hourly basis, the median profit was $3.37 per hour".
Because actual vehicle operating costs are significantly lower than the IRS allowance of $0.54/mile, many drivers report incomes that are substantially lower that their actual incomes, leading to a large pool of untaxed income (although it is small for each driver).
Techcrunch has a summary here. [techcrunch.com]