Ever since the social networking site of Facebook opened itself up to outside development, there has been a flurry of Facebook applications created by independent programmers and companies.

Facebook Platform is an application programming interface that allows anyone to make applications that integrate with, or run within, a user's profile page.

The selection of applications that a Facebook user can download varies from the relatively simple, like slide-show viewers for displaying snapshots or complex programs including games and even a Nintendo emulator.

Since Facebook Platform launched in May, more than 5,000 applications that run on Facebook have been developed. And the number is steadily growing by the day. A big reason why this has been happening? Money -- or the hope of it, to be exact.

It seems like anybody who is curious about making a Facebook application is looking to become rich from doing so. When a venture fund claims that a Facebook application that helps to determine a user's "stripper name" is worth US$2 million, the question becomes not "How can an application on Facebook possibly generate any revenue?" but rather "Hey, what do I have to do to get a piece of that?"

Facebook doesn't permit an application to push advertising when it's run on a user profile. It does allow for developers to include advertising on their applications' "canvas pages," which are the home pages for applications on Facebook.

Thus, the simplest way of trying to make money from one's Facebook application is to put advertising links or banner ads on its canvas page. Some even sell ad space on their application's canvas page to their fellow Facebook developers. The creator of a application might want to buy ad space on the canvas page of a popular application in order to promote his new application, for example.

Money to be made?

Yet some developers doubt they can make much, if any, money from their Facebook applications. You can count Sidney Price as one who feels that way. He co-created My Room, which allows Facebook users to furnish virtual rooms and populate them with avatars. My Room has more than 80,000 users, but Price doesn't foresee becoming a dot-com 2.0 millionaire off of his popular Facebook application.

"Developing applications to make money is not effective," Price insists. "We understand the potential, but most people see through that. There are plenty of failed Facebook groups focused on branding that are proof people are not interested in advertising. With the variety of applications available, individuals would rather spend time using something that focuses on enjoyment."

On the other hand, there is Flixster Like Facebook and MySpace, Flixster is a social site, but one geared toward the movie aficionado. The site created a Facebook application, simply titled Movies (requires Facebook registration to view), that quizzes Facebook users on their taste in movies and compares the results with those of the people on their friends list. The Movies application has more than 450,000 users, and its popularity has helped to route more visitors to Flixster's own Web site.

The Movies application starts out by testing your likes and dislikes of popular movies so you can compare your tastes to those of others.

Steve Polsky, president and chief operating officer of Flixster, is bullishly optimistic -- he believes his site can build a profitable business on top of its Facebook application. He scoffs at the question of whether the frenzy to make money off of Facebook applications is simply hype.

"We see Facebook as a very strategic platform for Flixster," he says. "For Flixster in particular, the movies vertical is one where studios spend significant advertising dollars promoting their films and are continuing to look for ways to advertise more directly to their audiences. Flixster is already in the process of building a large business through our Facebook application, enabling studios to directly target their movie fans."

Other developers feel it's too early to gauge whether there is serious money to be made from Facebook applications. "I'm trying to remain cautiously optimistic," says Blake Commagere, who has helped to create several Facebook applications. "I'm not as bold as some of the people that will claim 'Application XYZ is worth $10 million.' I won't dispute that there is value in a lot of these apps. But $10 million is a lot of money."

Price says he doubts that his My Room application would have become popular had he and its co-creator devised it from the beginning to make money. "We do not believe that if it was ad-based and focused on generating revenue we would have achieved the growth we have. We have considered ways to generate revenue, but currently we are consumed with the effort to make our users happy," he says.

His advice to others interested in making a Facebook application? Build something for the love of it. "In the end, receiving e-mails from users who share your same love for what you built is very satisfying," he says.

Third-party developers offer the following tips on how to make a popular application for Facebook:

1. Target the friends list

One of the biggest draws of developing an application for Facebook is the unprecedented access to the social networking site's massive user base -- its so-called "social graph." Developers can reach a broad audience with very little effort.

"Typically, software developers struggle to get anyone to use their products, regardless of how useful," says Haroon Mokhtarzada, CEO of Freewebs, which created the medieval fantasy game Warbook (requires Facebook registration to view) for Facebook.

So the developers of the most successful applications say to exploit the social "interconnectedness" of the Facebook user base. It can be used to target the demographic you want to "sell" your application to.

"Facebook doesn't give you all the different social graphs of data that it has," Commagere explains. "The friends list is the most readily accessible one, but many others can be calculated. I could construct a query against Facebook servers that determines 'friends that love movies.' I can do things with the results of that query, like encourage [the user] to interact with these friends through an app that features movie-related stuff, like 'Send a movie quiz to these three friends because they love movies like you!' What you can typically query is always a subset of the friends list."

2. Make it sticky

Create an application that encourages people to interact with it frequently -- something that is useful, fun or addictive by design. Applications that are "set and forget" (such as one in which the user simply puts together a list of favorite friends, movies, music or other items) can quickly fall into disuse.

"Growing daily active users is far more important than total users," Mokhtarzada explains. "Utilize Facebook's social graph to make the application interesting to people."

3. Make it social

Price, who works as an interior designer, says he loves how Facebook brings people together. So he and a friend who is also an interior designer created the My Room application to exploit the social aspect of Facebook. "As interior designers and developers, we wanted to create an application that would bring together those who share a love for design," he says.

Likewise, Flixster created Movies to bring together people who love movies. "Flixster is all about a person's personal relationship with movies, sharing, rating, reviewing and discovering movies through friends and friend recommendations. Facebook is the perfect environment for our application, where so many people are already connected with their friends. We have seen our application taking off and working so well in that environment," says Flixster's Polsky.

4. Leverage your site

Design an application that extends the functionality of your own Web site, thus, helping to promote your site. This is what Flixster did with its Movies application.

Another example: The Facebook game HoboWars is a "lite" version of the original Web-based game. Initially, the Facebook version was created in response to players wanting to show their player statistics on their Facebook profiles. Andrew Hogan, the game's creator, decided to take things a step further and made a self-standing HoboWars game that runs on Facebook. Though he has not been actively promoting the original Web site version of HoboWars, the Facebook version of the game managed to attract an additional 9,000 users to the HoboWars site.

5. Prepare to scale

If your application grows in popularity and use, be ready to invest in more hardware or more server bandwidth, warn experienced Facebook application developers. What little money Hogan has managed to earn from his HoboWars application has gone back into buying new servers to handle the increasing load of users playing the game. He released his Facebook application on Sept. 10. Within two weeks, HoboWars hit 70,000 users a day. "That's when there were problems," Hogan says.

Commagere knows first-hand how having a popular application can bring about hardware issues. His trio of monster-themed games -- Zombies, Vampires and Werewolves -- have been consistently ranking high on the list of most popular Facebook applications. "If things take off, scaling can be stressful, but take solace in the fact that everyone goes through those growing pains," he says.