Nguyen Tung Lam, a 16-year-old high school student in Hanoi, uses Japanese mobile messaging service Line to chat with his girlfriend because she "likes the cute icons such as the teddy bear and bunny".
Doan Nguyen Trang, another Vietnamese teenager, prefers South Korea's KakaoTalk app because it is promoted by a wildly popular Korean boy band.
"I use KakaoTalk because Big Bang also use it and they are number one; I love them," says the 14-year-old.
KakaoTalk, Line and WeChat, a mobile messaging app developed by China's Tencent, are spending tens of millions of dollars on television advertising, online promotions and celebrity endorsements as they fight for the attention of tech-savvy southeast Asian teenagers.
(Read more: Why most mobile apps can't be trusted)
With a population of 600 million people, a burgeoning middle class and fast-rising smartphone sales, southeast Asia has become the front line in a battle for mobile phone users that is threatening the traditional dominance of mobile phone network operators, global internet companies such as Facebook and Google and now-struggling handset maker BlackBerry.
Like their western rivals, KakaoTalk, Line and WeChat allow users to send free messages through mobile internet connections but their playful, teen-friendly style has set them apart, driving them to the top of many app download charts.
"When you use Asian mobile chat apps such as KakaoTalk or Line, you have a certain sense of joy and fun communicating with your loved ones, whereas western apps focus more on pure functionality," says Le Hong Minh, chairman of VNG Corporation, Vietnam's leading internet company.
(Read more: Yahoo's acquisition spree all about tackling mobile)
KakaoTalk sparked the Asian mobile messaging revolution when it launched in 2010, but it has been overtaken by Line which this month crossed the 200 million user threshold, just two years after its inception – an accomplishment that took Facebook and Twitter more than five years.
"Facebook and Google definitely see these mobile messaging apps as a threat," says Mark Ranson, an analyst at technology research company Ovum.