This is gross abuse of candidates who are looking for work in a highly competitive market. Getting hundreds of candidates, only one of which will ever be paid for their hard work, to get more customers for Uber is another example of how corporations are taking advantage of people at every turn. Not to mention those hunting for jobs are often a vulnerable and possibly desperate group when out-of-work.
This is not about Uber getting the best and sharpest candidates with large social reaches. This is about Uber getting the cheapest labor possible: free. They'll probably be able to easily pay for the first year of their new marketing employee through this scheme.
I'll be thinking twice next time I need a car service, and I may put the Elvis karaoke taxi in my phone instead.