Uber has broken a lot of rules in its 10-year history. So why should its debut on Wall Street on Friday, in the biggest US tech flotation since Facebook, be any different?

Even by the standards of ambitious, lossmaking technology start-ups, burning through US$2 billion ($3b) in cash a year this late in its existence seems wildly extravagant.

But in their meetings with investors in recent days, the company's bosses gave no indication when they expect to make a profit.

Instead, they sought to sell the ride-hailing company as a once-in-a-generation opportunity: a chance to back a business with the