Kakao Talk/iTunes
KakaoTalk launched March 18, 2010 and gained by 57 million registered users by August 2012 while it jumped to 130 million registered users as of December 2013.
According to The Chosun Ilbo, the app sends and receives 3 billion messages per day. On top of that, Mashable reports that KakaoTalk is expected to earn $200 million in revenue for 2014. As part of their strategy for the new year, the company plans on expanding to emerging markets like Indonesia, Vietnam and the Philippines.
The app is available for iPhone, Android, Blackberry, and Windows.