There's a reason WhatsApp is valued at $19 billion.

The question of the week: How could a messaging app possibly be worth $19 billion? It seems that Facebook is betting big—huge—that consumers are looking to “communicate one-on-one or with very small groups, rather than sharing information more widely,” as The New York Times reports today. Younger users, especially, have reportedly become more enamored of other platforms.

Messaging apps are among those, and they’ve seen explosive growth over the past year, as GlobalWebIndex reports. Its figures show that for WhatsApp—the biggest player, but far from the only one—usage among 16- to 19-year-olds grew 160 percent last year. In the Middle East and Africa, as much as 69 percent of the teen mobile audience uses WhatsApp. Overall, as this chart shows, usage of WhatsApp grew by an even larger 175 percent globally in 2013. And in contrast to Facebook’s slowing growth in North America, WhatsApp increased its users in the region by 230 percent—although in terms of user numbers, North America still represents a small fraction of the global total.

Deloitte forecasts that in 2014, messaging services will deliver some 50 billion messages a day globally vs. around 21 billion SMS messages. But these apps are also becoming much more than just messaging tools—as we point out in our 100 Things to Watch for 2014, they’re evolving into retail channels as well. Japan’s Rakuten, an e-commerce giant, just made a $900 million deal for the messaging app Viber. Some apps, like China’s WeChat, also offer payment functions.