Uber CEO Travis Kalanick at Fortune's Brainstorm conference in June 2013. Flickr/Fortune Most of the data spans only one month, December 2013. Even though the numbers are dated, there's a lot to learn from it. For example, San Francisco generated nearly $18 million of revenue that month. (Uber's revenue is the total amount it collects from riders, most of which Uber then pays to drivers.) A year o