WhatsApp has reached a milestone as it moves closer to joining an exclusive club of popular mobile apps. The communications network now boasts 900 million monthly active users, WhatsApp co-founder and CEO Jan Koum announced via a Facebook post Thursday.

The public post simply read, “WhatsApp now has 900 million monthly active users -- with Mark Zuckerberg at Facebook Building 20.” While the message was short, it indicated a significant step forward for the app, which is part of an industry competing for reach and retention of users. Messaging app WeChat boasts 549 million monthly active users, and Line has about 200 million.

whatsapp koum WhatsApp cofounder Jan Koum announced his mobile app's new user growth numbers via a Facebook post on September 3, 2015. Photo: Facebook Screenshot

WhatsApp, which Facebook purchased in February 2014 for $19 billion, also competes with Facebook itself. The network typically touts 1.49 billion monthly active users. Last month, Facebook CEO Mark Zuckerberg announced that 1 billion people had simultaneously been on the main social network. Facebook Messenger, which split from Facebook as a stand-alone app in 2014, is up to 700 million monthly active users.

WhatsApp itself has been growing. At the time of Facebook’s acquisition, WhatsApp had 450 million monthly active users. In July, Zuckerberg reported that there were 800 million. That number is expected to increase as the company adds more accessibility across devices. Last month, WhatsApp opened a Web service -- WhatsApp Web -- to iOS clients. WhatsApp for Android also added "mark as unread," custom notifications, muting chats and emoji skin-color variations.

whatsapp-numbers In Facebook's second-quarter earnings report, the company broke down user statistics for each of its mobile apps and sites. As of September, WhatsApp is now up to 900 million monthly active users. Photo: Facebook

Facebook Chief Operating Officer Sheryl Sandberg congratuled the WhatsApp team via a Facebook post. "900 million people now use WhatsApp every month to stay in touch with family and friends around the world. Congratulations to Jan Koum and everyone at WhatsApp on reaching this incredible milestone -- and for still finding time to make us all laugh," she wrote.

Despite all the growth, Facebook has yet to find a way to fully monetize WhatsApp. The app, which allows texting and voice calls overseas through Wi-Fi and data, does not carry advertising, unlike its parent company. It does charge users a $1 annual subscription fee after the first year. That fee brought in about $15 million in the first half of 2014, TechCrunch reported. The company has yet to issue a further breakdown of the financials.