NEW YORK Last April, Reebok kicked off its "Run easy" campaign, using print, out-of-home, TV and online to promote the shoemaker as the everyman alternative to the catering of hard-core athletes by rivals like Nike. Three months later, the traditional elements of the campaign wrapped up -- except Goruneasy.com, a community site Reebok agency Carat set up for the push. There, runners kept coming back to share routes and post messages. Nine months later, they're still coming -- even from places like Australia, where Reebok never ran ads.

The extended life of Goruneasy.com presents Reebok and Carat with the opportunity to continue reaching consumers on their own terms. Where previously a campaign site might fade to black when the media support ended, Carat is keeping the site going -- even to the point where it's considering selling ads on it as a media property in its own right.

The splintering of digital media, particularly driven by the primacy of search, along with the rise in social media, is calling into question the linchpin of advertising: the traditional flight-based campaign. As marketers increasingly look to provide consumers information on their terms, some see a shift to an "always-on" marketing approach rather than campaigns defined by a strict beginning and end.

"We've got people coming back -- that's loyalty," said Sarah Fay, CEO of Carat, part of the Aegis Group. "One of the most important things a brand can do today is for people to come back and spend time with them."'

Even promotional efforts can take on a longer shelf life. Take Sheraton Hotels and Resorts, which last week launched Sheratonwave.com, a National Collegiate Athletic Association basketball tournament promotion that challenges people to submit videos of themselves doing the wave, enabling visitors to watch different wave clips stitched together. While the campaign is slated to end at the end of the tournament (April 7), Sheraton's digital agency Avenue A/Razorfish has contingency plans to keep the site running should it catch on with users, since talk about college basketball does not end with the final game of the season.

"The traditional campaign model doesn't work anymore," said Shiv Singh, director of strategic initiatives at Avenue A/Razorfish, owned by Microsoft. "If you have a social-media driven campaign, you can't stop it necessarily when you want to stop it. It's akin to having a dinner party and suddenly turning the lights off."

Many social media campaigns fail because they follow a media model of running for a set period of time, said John Battelle, founder of Federated Media. Speaking at an Ogilvy & Mather conference last week, he argued that a campaign like last year's Chevy Tahoe consumer-generated media effort that resulted in many consumer creations pointing out the Tahoe's poor fuel economy might have succeeded if it had kept running, as Chevy has since come out with environmentally friendly models. Instead, it missed being part of an authentic conversation about the environment, and how families who can't pile into a Prius might find the Chevy attractive.

"We ask marketers to think in flights and campaigns, but conversations don't happen in flights," he said.

It's not just in social media that a time-based campaign mind-set is giving way to an on-demand mentality. Google is consulting with several top clients to help them move their internal marketing systems to support what it calls a "portfolio-management" approach to marketing that has all corporate assets digitized and available on demand. Google has succeeded, in large part, in doing this in search, where its system handles millions of company assets (in the form of text ads), then responds to consumer demand arising from Web queries. Advertisers set budgets that Google's algorithms match with consumer demand, ensuring they remain always in the market as consumers seek out information. After closing its deal to acquire DoubleClick last week, it can move ahead to extend this always-on marketing into forms of assets beyond simple text ads, including display, video and audio. What's more, thanks to behavioral targeting, advertisers are increasingly able to reach discrete audiences, meaning their budgets can go farther.

"As targeting gets more refined, marketing will be more efficient and the mind-set will shift to serving key audiences on a more continuous and on-demand basis rather than push messaging," said Jeff Marshall, digital managing director at Starcom USA.

Of course, Google executives go even farther. As long as the matching of customer demand to advertiser is right, Penry Price, Google's director of North American sales, said, "the budget is almost irrelevant."

That will require shifts in how advertisers and agencies approach their marketing plans, said David Kenny, CEO of Digitas. Promotions and flighted campaigns are not going away, though some will increasingly get funded after always-on aspects, which tend to relate more directly to sales, he said.

Always-on marketing will also put pressure on agencies to use technology to create different ad executions that are rotated constantly and tweaked based on their performance in the market. While 80 percent of campaign work tends to take place at the beginning of the effort, that will shift to during the marketing, said Rishad Tobaccowala, CEO of Denuo, a Publicis Groupe digital media consulting firm.

That's a far different creative process, noted Jean-Phillipe Maheu, chief digital officer at Ogilvy & Mather. "That means we have to be able to develop work faster," he said. "We need to be able to listen, react and respond to what engagement works and what doesn't work."

Continuous marketing, then, will require new tools that enable advertisers to mine data in real time to guide their activities. Google earlier this month took a stab at making this a reality with the introduction of a dashboard that gives advertisers the ability to see all of their marketing activities in one snapshot.

"We have to build the right tools, measurement capabilities and infrastructure to prove these things are accountable enough that there will always be funding for these interactions," said Price. "The technology isn't there yet."