Some 98 percent of smartphone users in Korea use KakaoTalk, the nation's most-popular free messaging app.

It's a similar story in Japan, where over 90 percent of smartphone users subscribe to a single messaging application, Line, created by NHN Japan, a subsidiary of Korea's largest web portal.

NHN launched Line over a year after KakaoTalk was introduced in March 2010, but it caught up quickly and outpaced its rival in terms of total subscribers last month.

As the two vie for dominance, Kakao announced earlier this month that it will focus more attention on Japan.