Uber has a clear business model with clear revenue and cost centers. In this way they're similar to Amazon, spending to establish themselves. Their biggest ch

Uber has a clear business model with clear revenue and cost centers. In this way they're similar to Amazon, spending to establish themselves. Their biggest challenge (beyond regulatory hurdles) is the race to the bottom, as their primary cost centers are their drivers, and there's only so far that they can drive down wages. But there's a whole range of Valley businesses out there with the same basic model: attract lots of users (preferably by being "social"), and then monetize them... Somehow. Where that somehow is almost inevitably advertising. How many of those really justify billion dollar valuations?