The instant messaging app Line is already dominating the lives of young smartphone users in Japan and has spread rapidly elsewhere in the world, but its developer is eyeing even more aggressive growth.

Unlike global giants like Facebook and Twitter, the nascent social networking service developed in Japan was created for smaller and closed networks. By downloading the free app to their smartphone, people not only can chat for free like with WhatsApp Messenger but also talk for free like on Skype.

What's more, and what obsesses young users, is the avalanche of stickers, called "stamps" in the Japanese version, which are like "emoji" on steroids.