word of mouth

まさにこれからのネット社会を表現した記事ですね。

In Mobile, Word of Mouth Is Powerful
So let's talk.
Marketing of any game is important: it's how you get the word out on your game. Without it, we wouldn't know about a ton of games. It's difficult to just plain discover something.

Sometimes, a marketing strategy can only do so much for a game. That's where word of mouth comes in. And according to a study presented by Gamasutra, when it comes to mobile games, word of mouth cannot be understated. It accounts for a large chunk of why people discover a game in the first place.

http://www.gameranx.com/updates/id/9812/article/in-mobile-word-of-mouth-is-powerful/

この表現は以前，先生に教えて頂いた表現です。word of mouthは「口上、口コミ」という意味(『新英和中辞典』研究社)があります。まさに口から口へと言葉が伝わっている様子を上手く表した表現ですね。将来生徒に教えたい英語表現ですね。(EnDough)