WhatsApp today passed 900 million monthly active users (MAU), according to founder Jan Koum.

The popular mobile messaging app most recently hit the 800 million MAU milestone in April.

Koum tagged Facebook founder Mark Zuckerberg in the announcement. (Facebook acquired WhatsApp in February 2014 for $19 billion.)

And Zuckerberg hit right back with his own tag of the WhatsApp founder, first congratulating him…

…and then sharing a picture of Koum at Facebook posting the update via his phone:

Screen Shot 2015-09-04 at 10.44.44

Last month, a study revealed that as many as three-quarters of WhatsApp users are on Android, with only 22 percent using the chat app on iOS. But that number may grow, as the company recently rolled out its web app for iPhone users.

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 MAU in March.

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