(FORTUNE Small Business) – How exactly is a not-only-for-profit company supposed to measure success? There's no simple answer to this question, but here are three successful U.S. companies that try to balance social and economic gain.

ZIPCAR allows people to rent cars by the hour, where and when they want. As CEO Scott Griffith explains it, the company's mission is to reduce pollution and congestion by reducing the number of cars on the road. According to Zipcar surveys, 40% of the Boston-based firm's 26,000 customers have given up their cars since they signed up for the service.

STONYFIELD FARM manufactures and distributes organic milk products. The Vermont-based firm helps family farms by paying them premium prices for their milk. Customers seem impressed: Stonyfield's yogurt sales have grown at an annual rate of 22% over the past 12 years.

WINDOWS OF OPPORTUNITY (WoO) is a for-profit spinoff of the Maryland-based nonprofit CLEARCorps. WoO replaces inefficient, lead-painted windows in inner-city houses. Besides saving some 700 homes over $350,000 in energy costs, WoO has reduced the risk of lead poisoning for local children. — Matthew Phan