In the advertising world the effectiveness of a campaign scales or collapses based on its effectiveness. In other words, you can spend a lot of money with the wrong message and persuade no one. I’m not crazy about the phrase "advertising 2.0" but I am very pleased with the idea that word-of-mouth marketing = scalable persuasion. If the message you’re sending is valid, it’s magnified at very little cost. On the other hand, the wrong message can damage reputations. Welcome to the world of high-risk marketing!

ClickZ: Word of Mouth: Advertising 2.0 by Dave Evans of HearThis.com

Think of a traditional TV buy; its pure persuasive power is inherently non-scalable. Sure, I can add markets and increase frequency. But that doesn’t scale the campaign’s persuasiveness. It only scales the campaign’s footprint and budget. As a marketer, consider "persuasiveness" verus "awareness" and "budget," and ask yourself which one you’d rather scale. In the current tech-enabled information channels, getting through is half the battle. …

To truly persuade, you must engage. That means getting people to talk.
Compare this to word of mouth. If someone you trust recommends you try something based on a current need, you probably will. If five people you know recommend the same, you’re even more likely to. If 50 people tell you to try it, it’s no longer a suggestion; it’s a movement and you’re very likely to join it. Word of mouth offers persuasive scalability. It builds until everyone with an opinion is talking to anyone willing to listen.