The numbers were shared in a blog post by WhatsApp, which also said it was aiming to add another six billion people to the service.
"So even as we celebrate this achievement, our focus remains the same. Every day, our team continues to work to improve WhatsApp's speed, reliability, security and simplicity. We're excited to see how far we've come. But now, it's back to work – because we still have another six billion people to get on WhatsApp, and a long way left to go."
Facebook boss Mark Zuckerberg said the milestone was an important step "towards connecting the entire world".
WhatsApp unveiled plans in January to drop its small subscription fee and focus on building business communications through the platform, though not through advertising.