Bitcoin is a nonproductive asset that is only worth what everyone else is willing to pay for it. Uber is a company that has an in-demand service and generates cash flow off of that. The market cap of the company is the value of future expected cash flows. $18 billion seems a bit high to me personally based on an inability to turn a profit and a horrible business culture that may prove to limit growth, but nowhere near as overpriced as the Bitcoin market cap is.