Last month smartphone running app RunKeeper crossed the 2 million download mark, FitnessKeeper's co-founder Michael Sheeley told MobiHealthNews in a recent interview.

"That makes us the number one running app in iTunes and number one for Android as well," Sheeley said. "I don't believe anyone is growing as fast as we are with hundreds of thousands of new users every month. We also have a good retention rate -- for our paying users about half of them remain active each month," Sheeley said.

Sheeley contends that if FitnessKeeper can create the best running app they can leverage that model for other sports and areas of fitness, and make good on living up to the parent company's name. There is some "internal plotting" within FitnessKeeper to move toward "HealthKeeper," too, Sheeley said. (Those seem to be longterm, blue sky plans, though.)

That said, FitnessKeeper's strategy moving forward is important to the wider mobile health industry -- RunKeeper recently announced plans to integrate data from Zeo's personal sleep coach service and is working to help Zeo develop its API. Earlier this year RunKeeper added data from WiThings' WiFi-connected weight scale, WiScale.

Unlike personal health record offerings like Google Health or Microsoft HealthVault, RunKeeper built a sticky mobile application, grew it to 2 million users and began to aggressively build out its online portal. By adding data from complementary services and devices like WiScale and Zeo, RunKeeper may be in a class of mobile applications that will succeed where PHRs have largely flopped.

What's more, Sheeley said that his company sold some 30,000 $30,000 worth of WiScales within 24 hours of announcing it would be integrating weight data from the WiFi scales. RunKeeper users were partially motivated to use WiScales because maintaining an accurate weight measurement in RunKeeper helps the program to better calculate calories burned during runs.

In the coming months when RunKeeper officially offers integration for Zeo sleep scores, the app company and sleep coaching experts will explain the connection between healthy sleep habits and running, which may in turn drive sales for Zeo, but will certainly be that much more stickier for RunKeeper's core user base.

"While I can't give you any hard numbers, the distances for runs and frequency of runs among RunKeeper's users has gone up steadily since we first launched," Sheeley said. "In the very beginning, if someone ran over a mile, it was unique. Then a run of a couple of miles became the average. Now we see marathoners are using the app and site. We hear a lot of stories about people who started using our app and had never run before, but they 'grew' as runners with the app."

Sheeley said that it's a good sign that "higher level" of runners like marathoners are using RunKeeper. Many of those runners opt for more expensive and more accurate running devices like the ones Garmin makes, but Sheeley believes these runners are drawn to RunKeeper because of its online portal and because GPS capabilities on the iPhone and Android devices has gotten better in the past few years. Sheeley said that while iPhone's GPS may never be as good as the standalone devices, RunKeeper users can share their runs in real-time, auto-share them on Facebook and Twitter or on the RunKeeper site. The auto-share function (rather than having to plug in a dedicated device to a computer's USB port) is starting to outweight concerns over accuracy, according to Sheeley.

Facebook is, in fact, where most of RunKeeper's new users learn about the app, Sheeley said. The launch of RunKeeper's Android app also just about doubled the number of new users the company has added each month, Sheeley said.

"Twitter is good too," Sheeley noted, "but with Facebook users can share their running maps and the little map icons people share on Facebook get their friends interested in seeing the routes they ran" and in some cases interested in downloading the apps themselves, Sheeley said.

This week RunKeeper plans to roll out new features for its online portal where users can better manage their privacy settings. One feature will allow a runner's friends to watch their progress during a marathon, race or just practice run in real-time via a GPS map. The new site will enable users to pick and choose who can watch them, instead of making it completely private or completely public, which is about as nuanced as Facebook and Twitter are today.