Even stranger when you consider that the Taxi AND Limo business is collectively an $11B/year business. http://www.ibisworld.com/industry/default.aspx?indid=1951 So if this isn't bubbly stuff, Investors must at least be extremely bullish on Uber increasing its market share.
In addition to global market size possibly being bigger, I believe Uber is capturing a bigger market size by targeting low end cost spectrum. Case in point, I rarely use cabs but have used Uber on and off for rides to places. It may just be me but I believe even in this market alone, the market size is bigger overall.
Add in the fact that Uber is tackling on new mediums like local delivery and the market size gets bigger yet again.