Emerging brands in the U.S. count on Facebook to promote themselves online. In India, they also rely on a Facebook service, but one that doesn't cost them anything—WhatsApp.

According to Mary Meeker's annual internet trends report published last month, WhatsApp is the most popular Android app in India, followed by Facebook Messenger at number two and the core Facebook service at number five.

With over 200 million users in India, WhatsApp is how digital health start-up 1mg went viral, even without founder Prashant Tandon having to spend any money promoting it. Since its launch in 2015, more than 9 million people have downloaded his app, which helps users research prescription drugs and find the lowest prices.

"One of our users created a long WhatsApp message on what a public service it was, and people just started sharing it," said Tandon, a graduate of Stanford Business School who returned to India in 2009 to build digital tools for the country's health care system. "We didn't do any marketing to get where we are."

Facebook CEO Mark Zuckerberg took a massive gamble on WhatsApp in 2014, shelling out $19 billion for a wildly popular messaging service that generated almost no revenue. While WhatsApp is still early in proving out a business model, analysts at Pacific Crest Securities predict that in coming years each active user could produce at least $2 of revenue per month.