The product is a bit more commoditized than with eBay or Craigslist though, right? Above some reasonable level of service, a ride is a ride. Assuming that the friction for switching services for the two sides is relatively low, it seems like there's a clear path to a new competitor starting slow and building both sides of the market at the same rate.

But for eBay, the products people want vary vastly from each other. I want to buy something fairly niche, so to save time it helps if there's one massive site where all of the people on the other side of the market are all located.