The reason why WhatsApp became popular was exactly because it piggybacked on the phone contacts to build a network (using the user's phone number as account id)

The reason why WhatsApp became popular was exactly because it piggybacked on the phone contacts to build a network (using the user's phone number as account id) which made it as easy to use as SMS. The only requirement was to install the app. Of course the same does not apply for example to LinkedIn or Facebook.