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.

Although the numbers are exciting, there have been the usual debates about whether interesting ideas for the Web can become profitable. Seth Goldstein, cofounder of the hosting provider and ad network Social Media, says his company hopes to make money from applications in three ways: by advertising on applications’ home pages; by designing applications that users will pay to install; and by using applications to do marketing research, providing perks to users who answer polls. Bay Partners’ Deshpande says that Facebook applications will stand or fall with all the other Web 2.0 sites.

While the question of making a profit is still unanswered, some developers have faced more immediate problems. Craig Ulliott, who designed Where I’ve Been, an application that tracks a user’s travel history and plans, found himself struggling to host the hundreds of thousands of users who added his application in the first few weeks. Without income from the application, Ulliott resorted to asking for donations to cover his rising hosting costs before getting help from investors.

And Facebook, for its part, became concerned about the flood of invitations that users were receiving to add new applications. To prevent these invitations from seeming like spam, the company limited users to sending out 10 invitations per application per day.

Although applications range from simple greetings to systems that allow users to host websites within their profiles, Farmer says the fastest-growing applications come in two forms: those that are purely viral, in which the point of the game is to convince a large number of friends to play along, and those that add to functions already present on Facebook, such as the Free Gifts application, which supplements Facebook’s existing Gifts feature. Many applications seem like toys. Of the 2,300 applications now on Facebook, more than 900 are categorized “Just for Fun.”

Farmer’s own experience as a developer suggests that it may be difficult for more-complex applications to gain ground. His application, Bookshelf, which lists, shares, and searches books, movies, music, and games, topped out at around 5,000 users in spite of having received positive reviews early on. Farmer says he thinks applications that are more complicated than core features of Facebook won’t take off.

Dave Morin, Facebook Platform’s marketing manager, says that as the development of applications for Facebook matures, he hopes to see applications that are more deeply integrated with Facebook and provide mechanisms for deep and surprising forms of social interaction. Many applications, he says, forget to take full advantage of Facebook’s News Feed–a system that notifies people about what their friends are doing. A concert application, for example, could post an item in News Feed telling a user, “Ten of your friends are going to the Smashing Pumpkins concert. Do you want to go too?”

“Applications that enable you to have deep interaction with your friends are the ones that are becoming most popular,” Morin says.