Alternatively you can just search "uber invite" on Twitter and you'll find any number of people hawking their own referral links. There was an article earlier in the year about someone who gained tens of thousands of dollars in Uber credit through taking advantage of the referral system: http://www.businessinsider.com/blake-jareds-50000-uber-credi...
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.
Depending on the city the promotions may get more aggressive, they also have a Get $25, Give $25 deal for referring new riders.