In my experience (specifically to the Nike Plus) it's a combination of both. Users who get to 5 runs on their own is certainly an indication of interest, howev

In my experience (specifically to the Nike Plus) it's a combination of both. Users who get to 5 runs on their own is certainly an indication of interest, however by encouraging users to get to 5 runs, you get a follow-on effect of "wow, look at how much running I've been doing" and further engagement. It's really satisfying building up a history and continuing to contribute to it. It's like the Jerry Seinfeld advice to use a wall-planner to chart out your tasks. For every day you complete the task (e.g. going for a run), you put a big red X up on the chart. Then it becomes about continuing a streak, rather than a daily chore. Nike Plus is a fill-in for that wall planner (and works even better when competing with friends who can see and harass you when you break that streak).