Bodo Schwieger of Team Red consultants network of Berlin (the king of "seamless mobility") pointed out to me recently that Montreal has more carsharing vehicles than Berlin! And twice as many as Hamburg and Hannover!
So here's a crude comparison of the major carsharing cities. I think vehicles is the best way to judge the size of a carsharing operation, since it's the vehicle that generates the revenues. Of course, the CSO's don't provide service to every square foot (or meter) of the city so counting the entire population of a city is a little misleading - but better than counting the whole metropolitan area as some have done.
Montreal - 3,509,238 population - 238 vehicles
Berlin - 3,405,250 - 130 vehicles (includes Potsdam)
Vancouver BC - 2,149,788 - 90 vehicles
Philadelphia - 1,517,550 - 50 vehicles (22 locations)
Boston - 581,046 - 180 vehicles
Seattle - 574,211 - 131 locations
San Francisco - 750,040 - 60 vehicles (28 pods)
Washington DC - 558,891 - 179 (combining my best guess of Zipcar 82 vehicles & Flexcar 97 locations)
Portland, Oregon - 546,150 - 75 vehicles
Zurich, Switzerland - 359,000 - 239 vehicles (142 stations, 17 vehicles at the Hauptbahnhof alone)
Sorry, I couldn't get a vehicle count for Hamburg because of changes in ownership there. My apologies to any carshare if my estimates are off (let me know and I'll update the vehicles and population figures). What do you think?