My teenager has been telling Chuck Norris jokes. As in, "Chuck Norris' tears cure cancer. Too bad he has never cried." A quick Google check turned up many such witticisms -- and lots of lame or even disgusting lines -- about the tough-guy actor who, as far as I know, my son has never seen in any of his many roles.

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A marketer might notice this and decide the actor's brand is pretty strong these days among a certain segment of the male population that has pumped up his reputation through word of mouth, both online and offline. Well, a marketer did, noting all the buzz about Mr. Norris on Web sites, blogs and even online quotes of the day that were dedicated to him.

In February, the Mountain Dew team started airing a commercial showing two guys goofing with the actor's image only to have him show up on their doorstep, chase them down and take his own video revenge. "What really inspired our agency, BBDO, and us is Chuck's recent rise as a pop-culture icon with the current youth generation," said Nicole Bradley, at Pepsi-Cola North America.

Word of mouth marketing used to mean getting a mechanic's name from guys at the barbershop or discovering a good restaurant over the backyard fence. These days, it comes from a blog, a Web site, message boards and online social networks such as MySpace.com, and it has advertisers buzzing. Because the chatter is much easier to follow in this virtual world than, say, a conversation at the salon or neighborhood barbecue, marketers are tapping into the buzz and, in some cases, even trying to start it and then track the results.

"The sea change is that for 40 years marketers knew that word of mouth was important, influential, pervasive. But they saw it as something to be described and hoped for," said Lawrence Feick, professor of business administration in the University of Pittsburgh's Katz Graduate School of Business. "It is only in the last 10 or 15 [years] that they have thought about it as a communication tool that can (at least in part) be managed."

BzzAgent, an agency founded in late 2001 to get consumers talking about brands, recently made a presentation to the Pittsburgh Ad Federation on its network of 200,000-plus volunteer "agents." The 8-year-old company Nielsen BuzzMetrics has built a business tracking what people are talking about online. Almost 300 people were in New Orleans in mid-April to get basic training from the buzz-meisters (OK, my term) at the Word of Mouth Marketing Association formed in 2004.

The association defines word of mouth simply as the act of consumers providing information to other consumers. Word of mouth marketing, the group has said, is giving people a reason to talk about products and services, and making it easier for the conversation to take place.

On the marketers' side, that's an exciting concept. As consumers tune out traditional ads, companies need to cut through the clutter. "We're becoming more and more concerned about that issue," said Brian Bronaugh, creative director at Strip District agency Mullen, who paid particular attention during the local presentation by a BzzAgent executive and is exploring using the agency's network in the near future.

But why would consumers be interested in helping a marketing company do its job? According to BzzAgent, people sign up through its Web site because they like being first on their block to see new products and because there's something cool about getting a say in how companies market things. "It's being in the know," said John Bigay, vice president of marketing.

BzzAgent also gives North American participants reward points based on how many conversations they report, although it claims agents ranked rewards toward the bottom of their reasons for participating. The company recently launched a United Kingdom network that doesn't offer even that. The prospect of gaining "social currency" alone will be enough, the company hopes.

To make a match that works for both the consumer and the company, the agency has those who sign on as agents fill out questionnaires. The agency wants to know who plays casino games, does scrapbooking, gardens, loves motorcycles and gets dinner delivered. It matters less who they are than what they love to do.

Not long ago, recruitment began for a campaign to stir up buzz about 24-hour Claritin RediTabs Tablets just as spring allergies kick in. "Spaces are limited, so sign up soon!" said the initial e-mail query. In such campaigns, agents may get a sample. If they like the product -- even if they don't -- they may talk about it with friends. Then they report back on their interactions.

Over time, the agency learns which agents are the most active and what they are most passionate about. Clients might pay for connections to several thousand agents with strong ratings in a particular niche or even for tens of thousands with the right credentials.

Historically, the beauty of word of mouth referrals has been that they were believable. If a friend said a dentist was good, he probably really believed that -- even if the dentist rewarded him with a discount on his next cleaning. If a teenager wore new shoes, chances were she liked them.

"It's not that all consumers are trustworthy," noted Peter Blackshaw, chief marketing officer for Nielsen BuzzMetrics during a recent "webinar." "But stacked up against marketers, consumers win."

Will consumers trust messages from people who are part of a corporate network, no matter how open the relationship? A Northeastern University professor, using BzzAgent data, found last year 75 percent of people didn't care if the person discussing a product had some link to the marketers as long as they trusted the agent.

But when people somehow discovered later the agent was involved in the marketing effort, some were unhappy. They might direct those negative feelings toward the individual, the brand or the company that made the product, according to the research.

Several years ago, BzzAgent began requiring agents to disclose they are part of the network. Policing that seems tricky, but last summer the company said it had decided to expel up to 10,000 "pests," or people who violated its terms of service. The Word of Mouth Marketing Association, which BzzAgent helped found, has written a code of ethics meant to make disclosure the norm.

Still, the idea of word of mouth as a powerful tool has inspired advertisers to do everything from paying bloggers to dropping product names into their copy to hiring actors to stand in lines and chat up a retail store. Not even all the companies assembling online consumer networks have addressed the disclosure issue.

In December, the Federal Trade Commission staff responded to complaints by saying it would make the call on a case-by-case basis. The regulators said if a consumer didn't tell his friends he was being reimbursed for pitching a cell phone, that would be a problem because it would affect how much credibility they gave him.

Offering rewards to try to stir up chatter can have mixed results, according to Dr. Feick, who recently authored a study on the issue. The Pitt professor said people don't like to be perceived as selling out friends, although they feel less strongly about accepting rewards for discussing products with mere acquaintances. He said long-term rewards programs seemed more acceptable as are those that offer some incentive shared by both parties in the conversation.

Getting people talking is only half the job, of course. A guy named Elvis Presley generated buzz by appearing on the Ed Sullivan show years ago. His records sold pretty well. Did one thing directly affect the other? Or did it matter more that the girl next door played her 45 for the neighborhood? Hard to tell.

That's where companies such as Nielsen BuzzMetrics try to track the online chatter, identifying links between different groups. In early April, the company reported a 338 percent jump in blog references to American Idol contestant "Sanjaya Malakar" as compared with his February buzz.

By filtering the various references through its systems, the tracking firm found many bloggers didn't think the contestant should win, but beauty care companies were getting a free dividend from all the chat about Mr. Malakar's ever changing hairdos. He has since been eliminated from the TV show's competition.

While Nielsen BuzzMetrics didn't pull together a brand association map for Chuck Norris, the company's blog tracking tools found a steady level of chatter on the Internet.

The advertising industry's current fascination with word of mouth has a lot to do with the technology that makes such information easy to get, yet some things haven't changed as much as it seems. More than one study has found that at least 80 percent of word-of-mouth exchanges still occur when people actually talk directly to each other.