The Japanese Internet retailer Rakuten plans to pay nearly $1 billion for Viber Media Inc., the company behind the Viber app. It’s an interesting strategy which allows Rakuten to compete more closely with similar communications apps like Japan’s LINE, Tencent’s WeChat and KakaoTalk.

Viber took in $1.5 million in revenues and booked an operating loss of $26.5 million in the year through December according to WSJ. Viber has around 300 million users, mainly in emerging markets, so Rakuten estimates it’s gaining them at a cost of $3 each. Analysts said the big question is whether Rakuten can make money off those customers.

That seems to be a big question too – investors hit Rakuten stock hard when the company announced the deal – shares plunged about 13%, whiping off about $2 billion in value. I guess there’s a big challenge ahead.