Fred sez: "Social media is no different from all media. The number of people who at one point were interested in your content or service is not that meaningful. What matters is the number of people who engage with your content or service on a daily basis and how engaged they are. And RSS subscribers, Facebook app installs, and follower numbers don't measure that."

Susn sez: Fred is totally bottom line level of right. This is the active versus installed base of any application (as opposed to that other metric, page views for media).and the user to install or registration ratio is always easily 3 to 1, even less over time as people churn out.