It's super popular all over south-east asia. One of the reasons are the cute and fun "stickers" that can convey 80% of your daily emotions and situations. Stuck in overcrowded subway? there's a sticker for that.
WeChat is also super popular in SEA. So we have Line from Japan, WeChat from China, KakaoTalk from Korea as the leading contenders in Asia. You see TV commercials of the apps endorsed by celebrities. They are evolving from simple chat app to a full social platform with own apps and games. You can say they compete with Facebook. I overheard a teen asking his friend why he is still using Facebook. Social apps to the teens might not be utility, more like clubs. And they want to be associated with cool clubs of the day.
You can add Whatsapp to that list, in Hong Kong at least. It's the dominant messaging app here and the company claims 50% penetration, although it's not clear by what measure.
WeChat and Line are definitely growing though. Not sure about elsewhere, but they've both been marketing pretty aggressively of late with TV and print commercials, billboards, store promotions etc.
They're sorted into packs or sets, each set with 24-40 stickers. You swipe between packs, and then between pages of 8 stickers. It's quite fast in practice, and you get used to what emotions can be expressed in each set. They also structure the sticker pages so that happy is towards the beginning, sad midway through, etc.