WhatsApp & Facebook’s Messenger app was the company’s fastest-growing platform in 2015, chief executive Mark Zuckerberg said earlier today at the annual F8 developers conference, and is the second most popular app on iOS globally, just behind Facebook.

At last year’s F8, Facebook Messenger was said to have 700 million monthly active users. In January, that number hit 800 million. Now, a few months later, it has ballooned to 900 million monthly users.

Given the growing popularity and widespread use of messaging apps globally, that stat isn’t totally surprising, but it’s notable that the growth Messenger outpaced other Facebook properties like WhatsApp and Instagram. Zuckerberg also said that activity on Facebook Messenger and WhatsApp combined is now three times the global volume of SMS messages, at 60 billion messages a day compared to 20 billion.

Facebook also announced a bot platform for the Messenger app, which as my colleague Casey Newton writes here, will bring a new range of functions to the app, such as personalized news stories from CNN or the ability to order flowers through the app.