Love the idea, especially with some type of push integration as a motivation mechanism. The “parallel calendar” idea could also serve as a motivator too (e.g. “Friend A is on an 8 day streak. Keep up with them!”) Also, imagine if you could somehow tally the goals across the entire network and measure how effective the app really is in changing behavior. (e.g. “We have a 98% success rate in getting folks to stop smoking/go to the gym/floss every day/etc”)