MyFitnessPal, a San Francisco-based startup with 30 million users, is today introducing an API that allows users to pull their data from apps and services like Fitbit, Withings, and BodyMedia, and plug it into their MyFitnessPal profiles.

This announcement by itself isn’t anything new. Other services offer similar tools, and releasing an API that makes it easier for users to pump more of their data into the service is par for the course. MyFitnessPal differentiates itself not by features, then, but by scale.

MyFitnessPal has been, to put it simply, growing like a ‘roided-out bodybuilder. Since January, 1.5 million users have signed up for the service each month. To put that in perspective, that means that roughly half of the service’s 30 million users have signed up in the last year, and if growth remains steady the service could double its size by this time next year.

It isn’t that people are signing up for the service and letting their accounts languish, either. Many members are using the service every day to track everything that they eat or every minute of exercise that they could possibly log in a single day. The company says that users have entered almost 24,000 years of exercise into the system, and have burned a collective 90 billion calories. Saying that that’s a lot of calories would be an understatement.

Unlike other services, which require a subscription fee or a piece of hardware to unlock the full potential, MyFitnessPal is completely free. The company, predictably, makes its money off by placing ads on its website and Android app. MyFitnessPal is operating under the classic  “give it away for free and sell ad impressions” consumer-facing Valley product model, and it’s working. Co-founders Mike and Albert Lee say that MyFitnessPal is profitable, and that the company hasn’t accepted any outside funding to fuel growth.

This is the rare moment where I’ll conceded the point that free-but-ad-supported might be the best revenue model for MyFitnessPal. By giving its tools away and encouraging a community without requiring any subscription fee, MyFitnessPal has removed one of the key barriers to entry that prevent people from using other services: they’re stingy. Every time a $10 bill or $99 tracking bracelet-slash-pedometer comes around, people can say “Well, I can’t afford it. Guess that means I don’t need it!”

And, honestly, who can blame them? Staying in shape can get expensive – healthy foods, in my experience, tend to cost more, and proper equipment (dumbbells, a pull-up bar, etc.) can get expensive fairly quickly. Asking people to pay an extra subscription fee on top of the startup costs of eating better and procuring all of those fitness-related knickknacks can be a tough sell.

By going the free route, MyFitnessPal has been able to reach more people than it probably would have if it charged a subscription fee. This, in turn, has given the company more data and a larger community, making it more attractive to someone that wants to try the service and to advertisers that want to get in front of as many eyeballs as possible. By working all of these muscles (pardon the pun) simultaneously, MyFitnessPal can become – or remain – a heavyweight in the fitness arena.