More than 150 million people log on to Facebook every day. Many users share photos—or what they last ate—with old friends; others play Scrabble. Now, some are even catching a ride to school or work via the site.

Zimride, created two years ago by two entrepreneurial 25-year-olds (with the help of a $250,000 grant from Facebook), aims to lower carbon emissions and commute times by connecting carpoolers. “For too long we have driven our cars with empty seats,” the co-founders note on their Web site.

Although the idea of a ride share board is not new, the start-up is taking advantage of the booming social networking infrastructure already in place to make the process more efficient, and therefore more appealing. Users may even feel safer by reviewing the Facebook profiles of other drivers and riders before they ever get behind the wheel or ride shotgun.

The idea has caught on at 30 college campuses and a couple businesses, including Wal-Mart, each of which pays Zimride an annual fee. “Like Facebook and Zipcar, Zimride has modeled its business from the campus outward, hoping to establish a framework in tight-knit college communities before broadening its approach to urban America,” reports Greenwire. Zipcar has also recently teamed up with the new startup to match riders with car renters.

"The big dream," co-founder Logan Green told the Sierra Club, "is that there will be so many rides posted on this system that online ride sharing becomes a reliable form of transportation, where you can log on at any time, enter any city, and find a ride. Once it hits that level of reliability, it can really fill an important role in our culture." Green said he hopes this will help overturn the country's car culture, as well as its impacts on society and patterns of development.

Picture of Zimride co-founders, Logan Green and John Zimmer, courtesy of Zimride