lift.do (now coach.me) used to implement that in a dead simple manner. To monetize, they pivoted from a "chain tracker" w/ reminders and (optional) social props

lift.do (now coach.me) used to implement that in a dead simple manner. To monetize, they pivoted from a "chain tracker" w/ reminders and (optional) social props to a social and coaching solution, so I don't know if this feature is still there.