Twitter has 200 million monthly active users.
The WhatsApp Messenger, which is available for iPhone, BlackBerry, Android and Nokia, allows users to exchange messages without having to pay for SMS.
After providing the service for free since its founding in 2009, WhatsApp is gradually phasing in an annual fee of 99 cents.
Koum denied rumours that he is in talks to sell WhatsApp to Google.
"Our goal is to build a sustainable, independent company. We want to build a business," he said at the Dive Into Mobile Conference, where he also ruled-out abandoning WhatsApp's policy against running ads.
"Mobile and messaging is the only thing we do," Koum said, rejecting the notion of turning WhatsApp into a broader platform that would include games, advertising and other features.