I came to the idea based on my own apps' observation: every time a new app is launched, it get a few hundreds of downloads. Then, literally no new downloads. Yet for a pretty long period, There are always a few dozens of players play the app everyday. The question is, even a good portion of the users think the app is good, they just keep to themselves. On the other hand, some type of apps have the self-propagate ability, such as cartoonlize-your-own-photo. You surely will send the processing result to a friend as long as you use the app. How to make a game app self-propagate? "Tell a friend to install then get the reward" is a good idea, but how to implement it? Better requires no server, minimal overheads to both developers and users?