Eric Ding had grown frustrated with the toll that stagnant federal grant money was taking on his department. So at the end of May, the 24-year-old doctoral candidate in public health did something unorthodox: hat in hand, he logged onto Facebook. Ding, who conducts research at Harvard's Brigham and Women's Hospital, used the popular networking site's "Causes" feature and created the  group Support the Campaign for Breast Cancer Research. At first, Ding's group was drawing several thousand of the site's estimated 50 million active users. "Then it was 50,000 a day," recalls Ding. Today he's got more than 2.2 million subscribers. "Within the span of six months, we've created a critical mass of supporters," says Ding. "It's just amazing."

Facebook may be awash in for-profit cash right now--after all, Microsoft just paid $240 million for less than 2 percent of the company last week. But it's also quietly at the fore of a revolution in charitable giving. On May 24, CEO Mark Zuckerberg began allowing third-party developers to build custom "applications" for the site. Since then, some 6,000 applications have been built to enhance the user experience on Facebook. Many of the most popular new apps help friends have fun: you can play Scrabulous with your buds, buy them a virtual beer, broadcast your local weather forecast. A few, like Change.org, ChipIn and Firstgiving, help you change the real world. But Causes provides the best example of the power, reach and potential of Facebook.

Launched by Sean Parker (a Zuckerberg pal and partner in the Founders Fund, which owns a stake in Facebook) and Joe Green, the application allows Facebook users to tap into the latent viral power of their social networks for good causes. When a Facebook member joins the Save Darfur Coalition cause, for example, everyone in his network is alerted of his interest. Same for when he recruits another member to join or donates money to the cause. It's a kind of altruistic peer pressure. "We're putting tools into the hands of individuals to mobilize their friends and ultimately build social and political movements," says Parker, 27, who had a hand in launching Napster, the once-cool music-sharing site, and Plaxo, the viral address-book service.

Since the Causes application launched, 27,000 Facebook Causes have been created to benefit 12,000 pre-existing nonprofit organizations, charities, relief groups, churches and political parties. "The Facebook generation wears their causes like the way they wear their favorite fashion," says Tom Watson, publisher of onPhilanthropy.com. It's a statement of who they are, he says.

It's certainly not about raising huge amounts of money, at least not yet. The Support the Campaign for Breast Cancer Research cause has raised barely two cents from each of its 2 million subscribers; the Save Darfur Coalition cause, which has more than 600,000 members, has raised barely $45,000. But that may be beside the point, say those who have launched causes on Facebook. "We wouldn't necessarily gauge someone's value to the advocacy movement based on what they've given," says Save Darfur's Allyn Brooks-LaSure. "This is a powerful mechanism to engage this critical population. They inform their community, attend events, volunteer. It's not something you can measure by looking at a ledger." Even if you've given a pittance to Save<