Notifications have made WhatsApp what it is today.
“WhatsApp started off as an app that would display statuses in a phone's Contacts menu, showing if a person was at work or on a call.
In June 2009, Apple launched push notifications, allowing users to be pinged even when not using the app.
Koum updated WhatsApp so that everyone in the user's network would be notified when a user's status changed. This new facility was used by users to ping "each other with jokey custom statuses like, 'I woke up late' or 'I'm on my way.'"
Fishman said "At some point it sort of became instant messaging".
WhatsApp 2.0, released for iPhone in August 2009, featured a purpose-designed messaging component; the number of active users suddenly increased to 250,000.”