Facebook Platform, an application program interface (API) that developers can use to build applications within the social-networking site Facebook, has created a Silicon Valley gold rush. The API gives developers a great deal of access to Facebook’s social resources. Developers can build applications that fit in several slots on users’ profile pages; have access to information from users’ profiles, friend lists, and friends’ profiles; publish information through the News Feed on Facebook; and send alerts and requests. Users can add and remove applications with the click of a button. Since the launch of Facebook Platform in May, more than 2,000 applications have been made available on Facebook.

“Facebook is now a social operating system,” says Salil Deshpande, a partner at Bay Partners, which is funding Facebook application developers through its AppFactory program. “There are going to be applications on top of [this and other] social platforms that we can’t imagine yet,” he says. Although so far many of the applications developed are simple, developers can use their access to the Facebook network to make heavily personalized applications that draw information from the activities of a given user’s friends.

Of course, part of the reason for the explosive growth in the creation of these applications is that their target audience is, by its nature, viral and active. Facebook is made up of 30 million users who have frequent and meaningful contact with each other. When a user likes an application, she invites her friends, and these friends invite their friends in turn. The result is that an application can grow exponentially in popularity.

RockYou, a company devoted to developing widgets for social-networking sites, is one of the companies specializing in viral marketing. Lance Tokuda, CEO, says his company plans to make money by using popular Facebook applications to advertise less popular applications, collecting a fee for each new user the company’s advertising attracts. Tokuda says the service will be helpful to big companies that are designing applications as a way of advertising to Facebook’s demographic and that have discovered that their applications aren’t attracting enough users to allow them to grow virally. To increase his company’s reach, Tokuda says, RockYou has created its own platform within Facebook Platform: an application called SuperWall. Typically, a person wanting to use an application has to convince his friends to install it as well. SuperWall allows people to use participating applications even if they’re not installed at both ends.

So far, viral marketing seems to work. For example, RockYou’s fast-growing application Likeness, a game in which users compare themselves with friends and movie stars, is adding more than 8,000 users an hour, and it has added more than one million in the past week, according to the tracking site Appaholic. Jesse Farmer, who runs Appaholic, says that Web developers traditionally hope for around 10,000 users in the first week.