Almost every instant messaging application such as Telegram and WeChat have the username feature. People can be found and added via their username alone without any other parameters.

However, other instant messaging platforms (read: WhatsApp) require a mobile phone number and a mobile phone number alone. You cannot contact someone else on the platform if you don't have their phone number.

Read more: http://www.istylmagazine.com/news/technology/you-may-soon-be-able-to-search-for-people-via-push-names/