NAVER Line is also currently killing it in this space in Japan, Korea and other nearby places.. Their business model started as "free but sell extra stickers to send to your friends", but now they have critical mass it seems they're setting up some kind of social network.. But a piece at a time by making an app at a time. Their Photo sharing app is gaining ground, as is their "diary" (read: timeline) app.
Since Marchish this year I've found Naver's LINE to be really unreliable though. Messages delayed days or hours, etc. I'm surprised it's still growing if this is the case for more than just me.
