Recently I have been making more use of my Fitbit, and one of the social "games" I have been playing on it is the step challenges. Counting steps as a form of interactivity is a fairly old concepts, with the first pedometers showing up from Japanese manufacturers in 1985 (interestingly, Leonardo Da Vinci had envisioned a mechanical step-counting gadget centuries earlier!).
What I found unique about the Fitbit's spin on the "step challenge" concept is the virtual races you can hold with your friends. Users can pick an iconic expanse to walk across to race on like the Alps, Appalachian or Rocky mountains and can see realtime where their friends stand along these trails. The fitbit will (if permitted) use GPS tracking to figure out how much distance users cover, or utilize their gait and steps taken to generate a somewhat accurate representation of distance covered on these trails. Furthermore, walking these trails allows users to unlock 180 degree views of these locations and in-game "treasures" and unlockables.
The second and somewhat less obvious effect of this interactivity is that I find myself feeling closer to the people who do these challenges with me, regardless of the multiple thousand miles between us. The concept of catching up and being able to overtake your friends helps me feel closer to them. I am not sure if the developers realized this aspect of their product, but I think this is something special, and I see the potential in a game that makes you feel closer to people by moving relative to them.