It seems with the selfie because of the time taken for changes to show-up that people might get demotivated in the interim.

Have you considered doing the same for food instead ?

That is having people take photos of all their meals and automatically sharing with their friends, to encourage eating healthier (soft competition) and making people think more about what they eat because it'll be broadcast.

That would seem more conducive towards a "real-time" feedback loop.