SAN FRANCISCO — WhatsApp has reached 900 million monthly active users, cementing Facebook's dominance in mobile messaging.

Facebook owns the world's two most popular apps: WhatsApp, which it bought for billions, and its homegrown app, Facebook Messenger, which recently announced it has 700 million monthly active users.

WhatsApp founder Jan Koum made the announcement on Facebook on Thursday evening.

Monthly active users isn't the best way to measure activity on a messaging app. WhatsApp did not say how many messages are being sent each day, for example. But the growth is impressive. WhatsApp announced it had crossed 800 million in April. That is no small feat for either app: Smartphone owners spend more time in messaging apps than any other app.

Messenger is also surging in popularity. It's now the second-most popular app in the U.S, surpassing Google-owned YouTube, according to a comScore report.

By way of comparison: Twitter has a bit more than 300 million monthly active users. Instagram, the photo and video sharing app owned by Facebook, also has about 300 million.

Still, WhatsApp faces fierce competition from Asian rivals. And those apps are making money from games, virtual goods and other in-app wares, moneymaking opportunities that Koum has rejected.

So far Facebook has not tried to milk WhatsApp and it's still unclear how it plans to. In the first half of 2014, WhatsApp made $15 million from subscription fees on a loss of $232.5 million. WhatsApp was charging users $1 a year, with the first year free, before it was bought by Facebook.

Facebook's chief executive Mark Zuckerberg said during the company’s second-quarter earnings call that the company is not yet ready to turn on the moneymaking spigot with Messenger or WhatsApp.

Zuckerberg has said Facebook has "many clear ways" to make money from a product once it reaches one billion users.

"This may sound a little ridiculous to say, but for us, products don't really get that interesting to turn into businesses until they have about 1 billion people using them," Zuckerberg said in 2014.

One billion people, one out of seven on the planet, used Facebook on a single day in August .

"This was the first time we reached this milestone, and it's just the beginning of connecting the whole world," Zuckerberg wrote.