The users were pegged at 600 million in August last year. The figure makes WhatsApp one of the world's largest social networks, bigger than Twitter's 284 million or even the photo-sharing service Instagram, which counted 300 million users. Facebook itself, however, is still bigger, and tallies 1.3 billion users signing in every month.

Adding another edge to its service for Windows phone users, WhatsApp has secretly announced in an email to roll out voice calling feature in its instant messaging app very soon.

In a reply to a question asked by a Windows Phone user, WhatsApp support team indicated that the feature for voice calling for them is in developmental stage.

"WhatsApp Calling for your platform [Windows Phone] is still in the works," it said.

However, the email hasn't specified any  time-frame for its roll out for Windows Phone users.

"We cannot comment on any future release timelines but we appreciate your interest in WhatsApp Calling," the mail further said.