The Kimberly-Clark Corporation, the maker of Kleenex tissues, is joining forces with Meetup for an initiative sponsored by the company’s baby brands, like Huggies and Pull-Ups. The effort, aimed at mothers of young children, represents Meetup’s first foray into sponsorship. The site is known as a facilitator of offline meetings of computer users who share interests like politics or parenthood.

Kimberly-Clark is one of two marketers that have signed sponsorship agreements with Meetup, on undisclosed financial terms. The other is the American Express Company, whose Open division is collaborating on an initiative aimed at entrepreneurs and owners of small businesses.

The sponsorships are indicative of the increasing interest on Madison Avenue in a wide variety of online tactics that include social networking, blogs, virtual worlds, community Web sites, social media and word-of-mouth marketing.

All are part of what is known as Web 2.0, the efforts to encourage two-way conversations between brands and consumers rather than the traditional top-down method of corporations talking at — or down to — potential customers.

“Brands can network like people network,” said Kirk Cheyfitz, chief executive at Story Worldwide in New York, an agency that has worked for marketers like A&E Networks, Nestlé, Toyota Motor and Unilever.

There is, however, a danger for advertisers who turn up on a Web site like meetup.com or at the meetings that Meetup helps people arrange. If consumers perceive that Web 2.0 activities are being overly commercialized, they will decry the involvement of the marketers and mark down the value of the efforts to a big fat 0.

Kimberly-Clark executives say they are aware of those risks.

“We started with feedback from Meetup members and organizers as to whether they would want a sponsor and what they would find of value from a sponsor,” said Brad Santeler, director for media and relationship marketing at the Neenah, Wis., office of Kimberly-Clark.

“It’s very transparent,” he added. “We asked them what they wanted, and we’re providing that.” For instance, Kimberly-Clark is paying the monthly fees that organizers of affinity groups usually pay to Meetup.