Rakuten Inc., the Japanese online retailer controlled by billionaire Hiroshi Mikitani, is buying the Viber Internet messaging and calling service for $900 million as it moves into social networking.

Viber will provide a distribution channel for Rakuten's digital products, Mikitani said at a briefing in Tokyo yesterday. Rakuten also hopes to create a game platform on closely held Viber, which has 300 million users of its instant messaging and free Internet phone services.

The acquisition will help Rakuten compete with Naver Corp.'s Line service and Tencent Holdings Ltd.'s WeChat, which both combine instant messaging with shopping and gaming. Rakuten, which wants to have 70 percent of its transactions come from overseas by as early as 2020, recently bought stakes in social-network operator Pinterest Inc. and digital book seller Kobo Inc. as Mikitani tries to boost sales after missing analyst estimates for two straight years.