By comparison, the taxi & limo industry does $11 billion in sales across the entire US market.
For Uber to ever justify just this funding round, they'll probably need to become roughly the size of 1/3 the entire taxi business in terms of sales (assuming $3.x billion in sales and $600x million in profit off that base, and assuming a reasonable 20 to 30 pe ratio over time).
I'm guessing Uber is betting on selling customers a lot more than their current service offerings. There's no other way they can justify this type of valuation.
As of 2013, Uber was only generating $200 million in revenue for the entire year (according to the leaked numbers). Uber could grow crazy fast and still not justify this valuation for six or seven years. That's quite the long term bet being placed here.