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Activation

Telling users when friends/contacts use the app

2 examples

Examples

Signal

Signal

Signal inform users each time one of their contacts (sim card) joins the app. A simple reminder to prefer Signal to its alternatives. This is a trick that Telegram also used massively at its beginning.
Signal screenshot 1
Signal screenshot 2
Instagram

Instagram

Instagram sends notifications both when your contacts (Facebook) reach the network and when people that "you may know" create an account. An interesting activation trick, except when you have no idea who the person is.
Instagram screenshot 1