LINE is a cross-platform communication service and app, offered for free by Naver, from NHN Japan. The basic functionality allows users to send text messages and to make free calls with other users who have the app installed on their smartphones. The service launched just 13 months ago, on June 27, 2011, and its growth rate has been simply amazing, with 45 million users currently registered — 20 million of whom are in its homeland. The rapid growth of LINE is currently one of the hottest Internet topics in Japan.

The 20 million registered Japanese users already makes LINE a real challenger to domestic social-network giants Mixi, Gree and Mobage, which each have around 25-30 million users. International players Twitter and Facebook have around 20 million and 10 million Japanese users respectively, and it took each of them much longer to amass those numbers. As LINE requires a unique phone number for each registration, 20 million users equates to 20 million cellphones (out of a total of 130 million) — pretty impressive for a service that did not exist until early 2011.

LINE initially started as a text-messaging service allowing users to text-chat with other LINE users or have group-chats with multiple users. Once the app is installed it's easy to know which of your friends also has the app installed as LINE simply looks at the numbers in your address book and cross-checks them with registered LINE users. If a user has friends who are already on LINE, at registration the app will suggest friends to add to your LINE-friends network. The app also works on regular cellphones, meaning smartphone users can send LINE messages to Japanese feature phones (aka "dumb-phones," which still occupy 75 percent of the market).