Here's typical consumer app funnel. It applies to apps that do messaging (like GroupMe or DrawChat), but also posting content (like any app that ends with "-gram" a la Instagram and Cinemagram):
Your exact numbers might vary. With DrawChat, we found that 90% of people that land on the App Store page will install the app. The next step is to actually open the app, but only 90% of of the people who download your app will actually open it - I'm guessing that people are stockpiling apps for the impending apocalypse.