ZipCar can't do that because their growth is constrained by the need for expensive assets (cars). Even if they were popular enough to expand by a whole order of magnitude over the course of a year they would require a huge injection of additional cash from additional equity sales or lending.
As a result, it just isn't really possible for ZipCar undergo the huge upside that a web-only company can.
The difference with purely web companies is Zipcar has a lot of fixed costs, if they want to expand to double the amount of users they will have a ton of capex to double the amount of vehicles (yes, you can probably squeeze a few more hours of usage into a car, but like planes there is a fixed number of hours any given vehicle can be generating you revenue per month). Web companies also see infrastructure costs dropping over time while Zipcar sees the opposite (after accounting for economies of scale, cars are not getting cheaper year after year).