WhatsApp has one billion users

"As of today, one billion people are using WhatsApp.
That's nearly one in seven people on Earth who use WhatsApp each month to stay in touch with their loved ones, their friends and their family.
We are proud of this milestone, and we're humbled by the extraordinary ways all of you have used WhatsApp. Whether it's sharing vital information during natural disasters or health emergencies, finding a date, growing a small business, buying an engagement ring, or seeking a better life – we're honored to be a small part of what people are doing to make their lives and the lives of those around them better.
And yet, through all the progress we've made together over the last seven years, our mission has never changed. WhatsApp began as a simple idea: ensuring that anyone could stay in touch with family and friends anywhere on the planet, without costs or gimmicks standing in the way.
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 6 billion people to get on WhatsApp, and a long way left to go."

Source:  Blog post by WhatsApp, 1st February 2016

MAUs
Update - more stats here