Facebook made a deal to acquire the app in February 2014, at which the app had 430 million monthly active users people. In April 2015, the number has doubled to more than 800 million users, and in August it had 900 million.
And with Facebook Messenger itself isn't far behind, the social giant dominates the messaging app market, leaving others in the competition by a wide margin.
WhatsApp's milestone was posted by founder Jan Koum on his Facebook page. Facebook's founder Mark Zuckerberg was among the first people to congratulate him, and even posted a photo of him as he posted the status update.
Being the most popular messaging app on mobile devices, WhatsApp has been criticized for not having a viable business model to take advantage of this growing users. Jan Koum took a strong stance against putting ads in its app, and only cost users $0.99 for a lifetime subscription after the first year.
But getting revenue from users using WhatsApp its not Facebook's way of doing business. What it does is getting as much users as it can, conquering the market, and use that number of users to get more attention from advertisers.
With the two apps integrated starting a year after WhatsApp acquisition, WhatsApp is more like a straightforward messaging app for those that don't like distraction and prioritize speed and usability. While Facebook Messenger is more to people that want more features and capability.
In this form, Facebook is trying to become distinct. By giving its messenger services their own homes, Facebook is trying to reach a more diverse community.
Related: Facebook Messenger on Google Play: Reaching 1 Billion Downloads With Its Own Way
That business strategy has worked extremely well as the the social network and its messaging apps are now so popular that they account for a fifth of the time people spend on their mobile devices. With that many people using the services by Facebook, and with a billion logins a day, Zuckerberg said that he's proud of the community for the progress.
"Our community stands for giving every person a voice, for promoting understanding and for including everyone in the opportunities of our modern world."
"A more open and connected world is a better world."
"It brings stronger relationships with those you love, a stronger economy with more opportunities, and a stronger society that reflects all of our values."
"Thank you for being part of our community and for everything you've done to help us reach this milestone."
"I'm looking forward to seeing what we accomplish together."
A Strategy Criticized, But Succeeded
Facebook Messenger that is becoming the second most popular app in the U.S., has made it surpassed Google's YouTube, to come only behind Facebook itself. ComScore said that as of July 2015, 59.5 percent of Apple's iOS or Google's Android smartphone owners used Messenger at least once a month, outperforming YouTube at 59.3 percent.
Facebook separated Messenger to be its own brand, initially to Facebook's users dismay. Users did not download and use it, making it once not that popular until Facebook stopped allowing users to send private messages in the main Facebook app. This move was criticized, but the company's bet in "separating the big blue app" has paid off.
Excluding WhatsApp, average mobile users spend around 46 minutes on Facebook's apps per day. As a company that is worth almost $264 billion, Facebook is also increasing its number of users. With almost 1.5 billion users, Facebook is used by half of the three billion people who have access to internet.
With that huge numbers the company has amassed, Facebook is still having its share price dipped (4.8 percent, as low as $92.30 a share). Despite the enormous figures helped Facebook's revenues to increase by up tp nearly 40 percent in the three months to the end of June, as it sold more and more advertising to the companies, investors are anxious about Facebook's increasing costs.
Facebook employs about 11,000 people around the world, and this number is relatively small for a company of its size and value. With the company's spending increased by more than 80 percent if compared to the same period the year before, shareholders are nervous as Facebook is recruiting more people to develop products.
Further reading: Facebook In the 2015 Tech Industry: The Social King Among The Relentless Giants