Marketers used to spend the bulk of their money on TV ads, which ran for a few weeks or months at a time. But these days, because of the Web, the shelf life on ads is infinite.

That has caused some new challenges: Some Web-based creative projects can last for years, taking on a life of their own. They can even take on forms that were not originally intended by advertisers. And because many of these ads are aimed primarily at a narrow slice of the general populace, they can take a long time to approach critical mass.

Still, there's good reason for this shift. Much more chatter about ads and commercials has begun to take place online. By moving efforts to the Web, advertisers hope to harness consumer reaction and sentiment. Almost half of the discussion of this year's Super Bowl ads, for instance, took place on social media sites focused on movies, video games, and entertainment news, according to TNS Cymfony, a tracker of online buzz.

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