Apologies; I shouldn't have assumed everyone is familiar with Uber. They regularly run promotions to get new users; ordering free ice cream via the app, for example. In addition to that, they have a referral program whereby one customer refers a new customer, and they both get a free or discounted ride. My invite code is xqdr9 if you want to go for a spin.

Basically, they're spending up to about $50 for each pair of users engaging in that way. It's likely the users will eventually become loyal Uber customers, but Uber might not realize that return in the short-term. It's the cost of growing so quickly, and the money they've raised is essentially to fund this.