First of all, it depends if you have to commute to an office in some industrial park (even if it's only one or two days a week).
Then there's the question of where your friends live, whether you regularly do activities outside the city, etc. Zipcar and Uber definitely make it easier not to own a car. But my observation of people I know in SF who don't own a car is that they make ample use of both Uber and regular/short-term rentals. People can definitely get by but it's often by adjusting their lifestyles and activities in that you just don't do things that are hard to do.