Why WhatsApp wants to convince Americans to stop sending text messages

Since the start of this year, a series of advertisements have appeared on television screens and billboards across the United States, with ominous warnings to texters courtesy of WhatsApp, the mobile messaging service acquired by Facebook in 2014.

"I think I left the car unlocked, can you check?" reads a text message displayed on one of the billboards. The consequence, outlined next to the text bubble: "If your personal texts aren't end-to-end encrypted, it's not private."

In a TV commercial, a mailman hands already-opened letters and packages to outraged recipients, before telling them that "every text you send is just as open as your letters."

Those warnings are courtesy of WhatsApp, the mobile messaging service acquired by Facebook in 2014. While WhatsApp has grown into a formidable force since then, used by more than a quarter of the world's population, the platform's reach in its home market remains comparatively small.

Data shared with CNN Business by research firm eMarketer indicates WhatsApp had less than 63 million users in the United States as of last year, or around 19% of the country's population. That's far behind its audience in countries such as India, Brazil and Indonesia where it is among the most popular modes of communication. India alone has nearly 500 million WhatsApp users according to eMarketer, which is more than a third of its population and over half its internet user base.