WhatsApp creator Jan Koum today announced that the messaging service now has 900 million monthly active users.

First to offer his congratulations was the head of Facebook, which acquired WhatsApp last year in a $16 billion deal. "Congrats! Here's an action shot of you writing this update," Mark Zuckerberg wrote, posting a photo of Koum and his cell phone (left).

WhatsApp has been steadily rising for years, moving from 400 million active users in December 2013 to more than 500 million in April 2014. By January, the popular app counted 700 million-plus monthly active users.

One of the reasons Facebook purchased WhatsApp was its growing user base. "WhatsApp is on a path to connect 1 billion people," Zuckerberg said last year. "The services that reach that milestone are all incredibly valuable."

Facebook's own Messenger app topped 500 million users last year and 700 million this summer, likely a result of the forced migration to the standalone application. By June 2015, the app counted a whopping 1 billion Android downloads.

Facebook itself last week reached a new milestone: one billion people active in a single day.

For more, see PCMag's reviews of WhatsApp for Android and iPhone.