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 is an encrypted messaging app founded in 2014. Like in Whatsapp, the user can Signal to make one-to-one and group discussions, voice and video calls.
February 16, 2021
Telling users when friends/contacts use the app
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.