When you think of social media, sites like Facebook and Twitter probably come to mind immediately. But messenger apps have actually caught up to social networks in terms of users. And increasingly messenger apps are being used for marketing.

The leader in most of the world is WhatsApp (a pun on the phrase “what’s up”), with 1.5 billion monthly active users around the world.

That kind of market penetration can’t be ignored. But how can you effectively use WhatsApp for marketing? Like all relatively unexplored frontiers, there are equal measures of risk and reward for early adopters. Here’s our marketer’s guide to WhatsApp.

What is WhatsApp?

WhatsApp is a free mobile app that uses your phone’s internet connection to let you chat with other WhatsApp users, without SMS text message charges. The app also lets you share files and images, and supports free voice and video calls.

Its support for a wide range of phones has made it especially popular in areas with high SMS charges, including Brazil, Mexico, and Malaysia—where 60 percent of the population uses WhatsApp. In fact, it’s the most popular alternative to SMS in 109 countries, or 55.6 percent of the world.

While Facebook acquired WhatsApp for US$19 billion in February 2014, it’s been operating as a separate entity since then, and hasn’t yet seen the same marketing-friendly features as Facebook Messenger.