WhatsApp, the messaging service Facebook bought for $19 billion in 2014, hit a major milestone on Thursday. WhatsApp founder Jan Koum said the service now has 900 million monthly active users worldwide.

That means WhatsApp added 100 million users in the past six months, after announcing crossing the 800 million mark in April of this year. Thursday’s milestone also means that WhatsApp has seen a 50% increase since August of 2014, when it hit 600 million users.

To put the 900 million number in context, Twitter has just over 300 million monthly active users. Facebook Messenger, the social network’s other popular messaging app, has 700 million users. WhatsApp’s parent, Faceboook, just crossed 1 billion daily active users. Instagram, the photo sharing app Facebook FB acquired for $1 billion in 2012, has 300 million users.

It’s worth noting that monthly active users is a metric that many have questioned for accuracy. For example, a person who opened WhatsApp once within a month for a few minutes could be measured as an active user. A more reliable metric would be number of messages sent.

While we know WhatsApp is growing, it’s still unclear how the app is going to monetize this massive users base. Facebook CEO and founder Mark Zuckerberg said during the company’s most recent earnings that there are steps that company will take before monetizing both Messenger and WhatsApp, including seeing people organically interacting with businesses before turning on advertising. We saw a glimpse of this with the debut of Facebook’s new personal assistant in Messenger in August.

The success of Instagram could also be a sign of what’s to come. While Facebook has remained quiet on how advertising on Instagram is performing, Emarketer predicts Instagram will bring in $595 million in ad revenue this year.

For more on Facebook’s new virtual assistances, see the video below:

Subscribe to Data Sheet, Fortune’s daily newsletter on the business of technology.