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The new face of word-of-mouth marketing

From Warby Parker’s Instagram account.

FORTUNE — Chubbies makes “radical shorts for men.” Its enthusiasts are denizens of Chubster Nation, a realm of 18-25, sports-playing, college-educated “guys.”

According to the Chubbies worldview: “Pants are for work,” while shorts — their shorts — are for having fun, or jumping off rocks, or playing beer pong, or climbing Everest. They dub this ethos the “shorts revolution.” And this particular revolution will not be televised, but it will be Facebooked, Tweeted, and Instagrammed.

Since Chubbies launched in late 2011, it has only marketed itself via social media. Yet these bright, retro shorts now ship all over the U.S. and over 40 other countries. “We haven’t spent much, but the growth has been awesome,” says Tom Montgomery, one of Chubbies’ four founders.

So what’s the secret to successful word-of-mouth marketing?

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“People you know and respect online are the most important sources of discovery when you’re looking for new things to buy,” says Forrester Analyst, Zachary Reiss-Davis.

But word-of-mouth marketing isn’t new. During World War II, there was a drive in the U.S. to encourage more people to eat animal organs, saving the best cuts for the military. Psychologist Kurt Lewin discovered that women — the gatekeepers of the family diet — began to reconsider their opinion of sweetmeat through conversations they had with each other.

Then there are other classic cases like Tupperware, which shunned formal ads for word-of-mouth campaigns in the form of Tupperware parties. By gathering friends, family, and colleagues to advocate for their goods at informal home events, Tupperware grew into a household name.

What is new, according to Reiss-Davis, is that companies are building structured word-of-mouth campaigns that make it easy for people to recommend and discuss products with people they know both offline and via the social web.

Take Chubbies: Its customers frequently share photos and comment on each other’s short-wearing escapades, turning customers into highly effective billboards. “Being able to scale in such a short time would have most likely been impossible, or very different, without Facebook,” says Montgomery, adding that the opportunity for a clothing company to reach people in a visual way, is one of social media’s key strengths.

To build a community of advocates, you have to be able to talk to them. When four Wharton MBA graduates started eyeglass e-commerce operation Warby Parker in 2010, they wanted to sell their eyewear directly to customers. So it made sense to market to them directly too. “We don’t view social as advertising, but as a way of engaging with customers,” says co-founder and joint CEO Neil Blumenthal. And they mean it. Like Chubbies, Warby Parker customers post photos of themselves wearing their shades and specs en masse. Everything from the company’s “Class Trip” — a whimsical voyage delivering glasses across the U.S. — to their campaign to give half a million pairs of eyeglasses to people in the developing world, gives people something to talk about.

To be sure, building a social web community is a process. “It doesn’t build overnight,” says David Fudge, head of social media at men’s clothing retailer Bonobos, who warns companies not to abandon their strategy when they don’t see an immediate uptick. When it launched in 2007, its clothes were only available online, and it claims to be one of the first companies to advertise on Facebook (FB).

Reiss-Davis argues that the most successful word-of-mouth initiatives treat customers as ambassadors and give them either special access to products or connections to executives and interesting people at the company. This level of access sounds like hard work, but it pays off. It may take you more labor to set up a successful word-of-mouth campaign,” says Reiss-Davis, “but you’re not spending those advertising dollars, which tends to make up for it.”

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Bonobos has an in-house team manning social media feeds. Their job is to be as active and consistent on Facebook as possible, which means responding to the majority of Facebook messages and liking Bonobos-related posts. But beyond pure manpower, what kind of content do people want to share?

Bonobos’ Fudge focuses on visual, witty content that expresses a unique point of view. Take the company’s 2013 April Fools’ Day prank about a fake product called the “Girlfriend Jean.” The video garnered over 66,000 views on YouTube and sent record traffic to Bonobos’ website. It wasn’t a billboard in Times Square or a commercial during the Super Bowl, but it certainly got a message across to customers.