I am using the activity sharing with a couple of friends. Basically you get notifications as they complete workouts or earn achievements. You can mute them if you wish. Especially that one overachiever who always closes every circle and gets at least 200% on daily move goal
As far as motivating, it doesnt help my motivation. I work out every day and close my circles regardless of what the others are doing. I do send messages after certain people accomplish things like earning the Earth Day badge. Personally I have never used a fitbit so I cant answer that question.