I think this is a little bit of a trap. Sure the number of people on the service are a feature and almost a meta-feature since it usually enables or multiplies

I think this is a little bit of a trap. Sure the number of people on the service are a feature and almost a meta-feature since it usually enables or multiplies the fun of other features. However the real value is number of active people on the service (which you did write). Those people were enchanted before I, as a new user, showed up. Therefore there is some orthogonal benchmark of quality or delight is required. These features I believe are what make someone get excited and share your app which then makes it amazingly social and draw in "the good people"