WhatsApp today passed 900 million monthly active users, according to founder Jan Koum. The popular mobile messaging app most recently hit the 800 million monthly active user milestone in April. Koum tagged Facebook founder Mark Zuckerberg in the announcement. And Zuckerberg writes with his own tag of the WhatsApp founder in a posting, first congratulating him. Last month, a study revealed that as much as three-quarters of WhatsApp users are on Android, with only 22 percent using the chat app on iOS. But that number may grow after the company recently rolled out its web app for iPhone users too. In June, Amazon’s Kindle app integrated a share button for WhatsApp and Facebook’s Messenger. And another big milestone came in March, when WhatsApp hit 1 billion installs on Android, making it just the second-non Google app to do so.

By comparison, Messenger passed 600 million monthly active users in March.