Research firm On Device found users of the mobile Web in Myanmar prefer Viber to other messaging apps and Facebook FB -0.46%. On Device Research

Facebook may be the most popular social network in the developed world, but in long-isolated Myanmar, smartphone messaging app Viber is getting more traction among users of the mobile Web.

According to a survey released this week by On Device Research, some 79% of 577 mobile Web users said they regularly use the Cyprus-based Viber app, which offers free texting and phone calls. That was more than the 58% of those who said they used Facebook, the world’s largest social network, on their devices.

Meanwhile, just 10% of respondents said they use messaging app WeChat, owned by China’s Tencent Holdings, and 5% said they use global messaging leader WhatsApp, the company Facebook recently agreed to buy for $19 billion.

A mere 4% said they use Line, a Japanese app that is popular in some Asian countries.

The popularity of Viber in Myanmar underscores the fragmented nature of Southeast Asia’s smartphone messaging markets, where no single app dominates. Rival messaging app makers are localizing content and splashing out on advertising in their quest for new users in the region, which is home to 600 million people.

Viber, which Japanese e-commerce giant Rakuten agreed to acquire for $900 million in February, says it has more the 300 million users worldwide. The service allows free voice calls and messaging, and it sells stickers and credits to make calls to non-Viber users. It is also popular in the Philippines and Vietnam, though other apps are used more in countries like Indonesia, Malaysia and Thailand.

Talmon Marco, Viber’s chief executive, told The Wall Street Journal that his service likely has attracted users in Myanmar simply because it works better than other apps on “networks that are less than perfect,” referring to the country’s poor telecommunications infrastructure.

“Looking at Myanmar, we see extensive usage for both messaging and voice, though as with all other Viber markets, there is more messaging use than voice,” he said.

To be sure, only a small fraction of people in the impoverished country of roughly 60 million have Web access, let alone smartphones. On Device says Internet penetration is just 1%, and that mobile phone penetration is 10%.

The country’s government in 2011 began enacting a series of economic and political reforms after years of military rule.