Such social sharing + gamification systems are no different than Github contribution streak or StackOverflow awards for streaks etc. Those streak award only benefited the platform, while awarding us fake points and badges, the fitness streak rewards and social sharing benefits the users health so arguably has a stronger case for being gamified.
We can argue all day that people should want to do fitness to be healthy, not on how they look or other people see them or their fitness, but reality is that the social component of fitness is a big part for many people be it at the gym or in an app.
a) Analysis and tracking of your own personal goals. (Some of the tools are better than the stuff available on the device itself.)
b) Sharing and socialising some other activities.
You can be careful and only allow certain activities to be public but you'll make mistakes and eventually many people will just think "whatever, I'll just default to public and remember to hide the ones I don't want to be public" and then it's even easier to make mistakes.
Defaulting to "opt-in" is all well and good until a human makes a mistake.
I have the family exercise group on mute, lol