RunKeeper's app.

A casual mention tucked into the press release on Nike's latest ad campaign sparked some interest from The Next Web last week, as Nike revealed that its Nike+ ecosystem now has more than 18 million users. While that statistic certainly sounds impressive, it might be helpful to put it in some context.

Many fitness companies -- particularly device makers like Fitbit and Jawbone -- have never released user numbers, or haven't shared them for some time. Others, especially app communities like MapMyFitness and Endomondo, share user base numbers in every press release. Like all self-reported numbers, these should be taken with a grain of salt. What does "18 million users" mean? Is it how many people have downloaded the app? For startups with multiple apps, does it count them all separately (potentially counting some people twice)? Or is it the number of active, signed up members using the app or device on a regular basis?

Nonetheless, some numbers are out there. Here's seven fitness app companies that have recently shared or let slip the size of their user base. Read on to see how they stack up.

MyFitnessPal: 40 million users

MyFitnessPal, a free nutrition and fitness tracking website and app suite, shared a figure of 40 million members when it announced its $18 million funding raise earlier this month. At the time, the company said it was adding 1.5 million users a month. Back in October 2012, MyFitnessPal put out a press release proclaiming 30 million users. If the company's numbers are accurate, it dwarfs the other entries on this list and is still growing.

MyFitnessPal has an open API through which it's partnered with almost every major company in the space: Fitbit, Withings, Jawbone, Runkeeper, Runtastic, and Endomondo, just to name a few. The company began in September 2005 and has been turning a profit since its launch, only seeking outside funding for the first time this month.