This really gets down to one of the core problems of understanding social media and self-promotion – deciding what "good" means. It can and should vary for pretty much everyone, which is an absolute fact which is overlooked most of the time when you talk about various metrics.

For me, personally, comment engagement is one of the best metrics out there. Not just "number of comments left," but "number of threads in which there is multiple posts by a single commentor." Anyone can leave one comment but very few people engage in conversations; those of the people who either hate you enough or love you enough to really talk about what you're doing.

But that's just my metric.