The words were simple; the logic sound. So Mike Lee listened to his personal trainer when he said: "If you want to lose weight, you need to count calories."

Lee, at the time, almost a decade ago, was engaged in a last-ditch effort to lose weight as his beach wedding rapidly approached. The trainer helped. But when it came to logging the caloric-content of his meals, he was left to do a lot of research and even more guessing. Logging the imprecise results with pen and paper felt silly.

This whole rickety system didn't exactly satisfy Lee, who'd been working at tech companies (including Palm and the famous boom-bust case study for the past several years. He describes himself as a product-minded person who's been programming computers since age 10. So Lee, on nights and weekends, set out to build a solution--an online directory of nutritional information.

"It was almost like starting from scratch. I built the entire site by myself, just Googling like crazy, trying to do the most basic things, like list files in a directory," Lee says. "I did the design myself. It was ugly, to be frank."

But Lee says he didn't think much of his little side project, dubbed MyFitnessPal, until he started getting personal responses from individuals who'd found the site and started religiously using it.

"A few months after we launched, we started getting before-and-after photos from users," Lee said. "People loved it so much--even my first initial version that was so simple. I started working all day and nights and all weekend, just from the passion to make it better."

Lee quit his day job and hired another man with a serious passion for building useful products: his brother, Albert. Well, "hired" may not be the most apt term for the arrangement, which specifically entailed spending most waking hours developing MyFitnessPal's website and app--for free. Neither Lee nor Albert took a salary for roughly the first year of the company's life. (Both admit they were awfully lucky to have wives who worked steady jobs during this time.) The company makes money through in-app advertising but doesn't disclose revenue numbers.

"We were very scrappy. But we started growing just through word-of-mouth," Mike says. "There's no paid marketing. There's no weird viral tricks. We just stay focused on, how do we help our customers succeed?" 

And by that metric, the company is on a roll. This month, the products made by MyFitnessPal, which now take the form of apps and an API integrated into most fitness trackers, crossed a milestone: They have more than 75 million users. Those users have lost more than 180 million pounds. The number is a bit incomprehensible, so just think of it as the weight of 120,000 elephants.

Internally, the San Francisco-based company is growing, too. In the past year and a half, thanks to a healthy infusion of venture capital funding in 2013, it has grown from 40 to 100 employees. 

Turns out, a lot of those 100 employees channel their zeal for their work in the same way Mike and Albert--now 44 and 41--did in the company's early days. That's thanks to a 10 person "customer happiness team," which not only collects the success stories that pour in but also invites users in to share their stories in live staff presentations. They also distribute collections of the most inspiring stories in weekly emails to employees.

One such email, according to Mike, included a letter from a user who wrote in on the day that for the first time in 20 years she had stood up from her chair without needing to use her arms to push her up. "For her, that's such a huge moment," Mike says. "And for us, too."