In the early days of WhatsApp, one growth hack that the original founders of the instant messaging platform used was to convert mobile numbers into the identifier for a user. This avoided the need for setting up a username itself, thereby making adding friends and family easier by just accessing the address book. For the mobile-first part of the world, especially India, the need for an email address to sign up was done away with. It's one of the reasons for WhatsApp growing so rapidly in India.
Life has come full circle for WhatsApp, in that it's finally introducing usernames. I hurriedly blocked mine to maintain a consistent username presence across platforms.
