The service gained 100 million users in just the past four months. In comparison, Facebook itself has about 1.49 billion monthly active users.
The social media king bought WhatsApp last year for $19 billion, even though it has its own messaging service called Messenger.
The sector is gaining ground as younger users chat and communicate through mobile apps instead of text messaging.
Earlier this week, photo-sharing app Instagram added a messaging feature, which may increase its 300 million user base.
LinkedIn is also jumping on the bandwagon. On Tuesday, the site for its messaging service, making it more like a messaging app than an email service.