Despite Facebook’s vast, international audience and best-of-breed viral features, the site has yet to realize its potential to advance social change. My two co-founders and I conceived of Rethos.com out of criticism of the landscape of mainstream social networking websites and their apparent obsession with superficial content. Indeed, it remains doubtful that Facebook’s 60+ million members are going to super-poke their way to a more just and sustainable world.

But the Facebook platform is beginning to show its ability to mobilize in support of social and environmental causes, not because of features built by its team of several hundred in-house programmers, but because of the site’s open door policy towards third party software developers.

In April 2007, Facebook released an application programming interface (API) that allows independent web developers to create software applications that run directly inside the world’s fastest growing online community. A flood of more than 15,000 ‘third-party’ applications have since been launched on Facebook. Among the most popular apps are Booze Mail, a mini email program that encourages users to send each other ClipArt-like images of Long Island ice teas and Alabama slammers, and Hot or Not?, a sort of avatar slide show that allows users to rate each other’s hotness on a scale of one to ten.

Sound like a vapid wasteland of superficiality? Fear not. A small but growing number of developers are introducing applications aimed at making this world a better place. Here are a few examples.

Debates, Rethos’ own offering to the Facebook community, is a discussion utility that encourages users to spark and share debates on the social and political issues that matter most to them and their friends. The app has enjoyed a burst of viral growth since its launch only a few weeks ago, and currently boasts about 1,000 users. Among the hottest debate topics are Barack Obama’s preparedness for the White House and the ability of capitalism to be a framework for social change.

Since its May 2007 launch, Causes, a fundraising tool for non-profits, has enjoyed stellar viral growth. But despite the app’s more than 10 million users, its daily use rate is less than 1% and it has yet to produce significant revenues for its thousands of non-profit clients. I give Causes top marks for its viral marketing and user interface but it clearly has some homework to do if it wants to compete seriously with other web-based fundraising platforms.

I Am Green is the very clever work of serial developer Karel Baloun, formerly a senior engineer with Facebook. The app is a user-generated database of tips for reducing the ecological footprints of its more than 100,000 users. Among my “leaves,” suggested tips to go green, are to purchase recycled paper products and second hand clothing. I Am Green even boasts some celebrity users, including Senator John Edwards and Dr. David Suzuki.

As its name would suggest, Carpool helps inter-city travelers in search of a ride share. Currently at just over 300,000 users, the app is especially useful for Facebook’s original audience: college students in search of an economical and green way to get from A to B. Carpool was created by Zimride, a start-up that plans to expand its debut app and launch other applications aimed at helping people get green.

So there you have it. Social entrepreneurs may be a minority among Facebook developers today, but that seems to be slowly changing. As the novelty of Booze Mail wears off (as does its hang-over), users of social networks are looking for a more meaningful experience. Between the various offerings on Facebook and websites dedicated exclusively to social change, such as Rethos, socially conscious web junkies have an expanding online tool kit to help them reak havoc on the status quo.