Last Friday, 4/16, Foursquare celebrated Foursquare Day. The twittersphere erupted with celebratory “Happy Foursquare Day” tweets. And Foursquare gave out a special badge to those who joined the tweet-tastic party. 

Mashable used it as an opportunity to write about the startup’s meteoric rise. At nearly one million users and 22 million checkins, Foursquare, which launched in March 2009, is growing at nearly twice the rate of Twitter. (Twitter took 2 years to hit one million users). 

But nobody seems to be asking the big question: why? 

What enabled Foursquare to grow so quickly? Ostensibly, it’s just another way to update your Twitter status with the added bonus of sending notifications directly to friends who want them. So why did Foursquare manage to race past Twitter’s initial growth rate?

The obvious answer is that Twitter itself provides Foursquare with an invaluable promotional tool by enabling the service to reach a broad audience outside its user base. Without Twitter, Foursquare would have taken more than twice as long to reach half a million users. But that doesn’t really explain why people start using it in the first place. What is the value of broadcasting your location by going into an application and selecting from a list of places in your area when you could just type in a quick tweet? What is the added value of making a location-based update via this application?

The secret of Foursquare lies squarely in its variable ratio reward system. Read: BADGES, MAYORSHIPS and other promos that it doles out at variable check-in count. 

In behavioral psychology, variable ratio reward systems have been studied to death—first by BF Skinner and later, a slew of scientists interested in the psychology of gambling. These systems came out of Skinner’s work in operant conditioning, a process by which consequences are used to change the occurrence and type of behavior. Skinner found the most effective and lasting form of conditioning is positive reinforcement, where rewards are given as a way to increase a type of behavior. Within positive reinforcement, the best way to get a behavior to stick is to reinforce it with a reward after an unpredictable number of times. Just like Foursquare, when it doles out badges after irregular number of check-ins. 

While Foursquare may have pioneered badge psychology, some of the most heavily used sites, games and apps also leverage this approach to encourage repeat visits. For instance, users on Farmville don’t know what it will take to pass a level or what kind of fun announcement they’ll get to make to their Facebook friends after a period of time playing the game. They just buy land, help neighbors and farm until something awesome happens. On Facebook, users never know whether a link or a status update they posted will attract comments or get some “thumbs up” from friends. But, every time a user gets some kind of reward for an action, it’s clear that it goes a long way to encouraging them to check-in, post a status update or play another hour of a game. (This is also why slot machines are so good at making money). 

In short, if you’re looking for a way to increase traffic to your site, grow a user base or get more followers, consider building in a system or variable ratio positive reinforcement. Ask yourself, what can we give away at random intervals that will get users to check back, comment or engage on a deeper level? If you’re at a loss for what you can offer, take a look around. There’s plenty out there worth imitating.