Facebook has decided to make its largest purchase ever with the acquisition of popular messaging service WhatsApp. Why did this messaging app fetch one of the largest sums in the web industry's recent history? Here are a few reasons.

The term windfall is hardly sufficient to describe the amount Facebook is paying to acquire upstart messaging service WhatsApp. The social media giant will pay $12 billion in Facebook stock for WhatsApp, along with $4 billion in cash and $3 billion in restricted stock for the app's 55 total employees and founders over four years. That's about 19 times more than what Facebook paid for Instagram two years ago in what was then seen as a risky, high-stakes bid.

It's a lot. According to Brian Blau, a Gartner analyst who spoke with the Washington Post, it's a stunning amount: "I am not surprised they went after WhatsApp, but the amount is staggering," said Blau. So why would Facebook pay at least ten times more for WhatsApp than what Google paid for YouTube or what Yahoo paid for Tumblr?

The short answer is global dominance.

The long answer is that Facebook's Mark Zuckerberg sees the future of the internet (and his company) in two linked markets: mobile and the developing world.

Mobile

Facebook has been investing in mobile for a couple of years, starting with its $1 billion surprise acquisition of then-startup Instagram a month before the social media giant's IPO. And after a rocky start as a publically traded company, Facebook has gained from its mobile platform, upending skepticism on Wall Street last year on the strength of its ever-increasing mobile ad revenue.

Since then, Facebook has doubled down on its multi-app strategy, recently introducing Facebook Paper -- the mobile fulfillment of the company's social news aspirations - and buying Little Eye Labs for $100 million, a small Indian startup focused exclusively on analyzing and improving Android apps. Of course, Facebook also tried to acquire mobile messaging service Snapchat for $3 billion, but no deal.

But while Facebook continues to dominate the social media world, bolstered by Instagram, Facebook Messenger, and its other apps, WhatsApp has exploded in popularity. Here's how Buzzfeed Business reporter Matthew Zeitlin visualized WhatsApp's growth, compared to Facebook messenger, Twitter, Gmail, and Skype.

And more importantly, the most of the ever-expanding user base of WhatsApp is using the app on a daily basis. While other apps like Line or ChatON are similar, WhatsApp is by far the most popular. According to Facebook's SEC filing for the acquisition, out of the over 450 million people using WhatsApp each month, 70 percent of those users are active every day. And it's growing quickly, with more than 1 million new registered users added per day.

"WhatsApp is on a path to connect 1 billion people," said Facebook CEO Mark Zuckerberg in the filing. "The services that reach that milestone are all incredibly valuable." Services that reach that milestone -- a rarity that only a few, like Facebook, have reached -- are also potential competitors.

Emerging Mobile Markets

The reason for WhatsApp's high user engagement rate is the nature of the app. It's basically one of the cheapest worldwide text messaging services. WhatsApp lets users text and send pictures to each other in real time across the world, avoiding international text messaging fees and only paying for data, plus a $1 annual fee, which is doable for most, even in developing countries -- especially considering alternatives like text messaging plans and phone cards. It also has a feature phone app, which is reportedly wildly popular in India.

WhatsApp does this without ads or frills; The messaging app is all about providing a core service cheaply and efficiently -- which is right up Facebook's alley with its plans to become the primary mobile internet service for the soon-to-explode markets in developing parts of the world like Asia, Africa, and Latin America.

Zuckerberg sees the future in emerging markets like Latin America, and wants to get in first, nabbing users new to the internet and securing Facebook as a known, trusted brand. Its been making moves in emerging mobile markets even before its IPO, with initiatives like Facebook for Every Phone -- a low-data feature phone version of the Facebook app - and with partnerships with wireless companies worldwide to help pay for users' data costs.

Twitter, Firefox OS, and messaging app Line are also trying to reach emerging markets early, but with less success than Facebook -- especially after its WhatsApp purchase.

That's because places like Latin America are where the internet is headed next. According to a recent Latin Post report, based on Gartner's research, while smartphones officially accounted for a majority of global phone sales last year for the first time, Latin America was the strongest growth market for smartphones in the last quarter of 2013, with sales growth peaking at 96.1 percent. While low-cost mobile device makers are making hay in Latin America, apps and services are the next step.

But is WhatsApp worth $19 billion -- more than Southwest Airlines, Nokia, or Tthe New York Times? There were plenty of skeptics about the $1 billion purchase of Instagram, but that appeared to be worthwhile in the long run. For WhatsApp, we'll have to see.