Everyone agrees that social media sites like Facebook and MySpace and twitter have garnered a lot of media attention. Indeed, one might say that they have been hyper-hyped. There’s little doubt that lots of people use them, but there’s also little evidence that they have marketing value.

A newly released study by Knowledge Networks found that 83 percent of Internet users between the ages of 13 and 54 participates in social media — nearly half on a weekly basis — but less than 5 percent use the sites for guidance on making purchase decisions across nine product/service categories. And only 16 percent of socnet users say they are more likely to buy from companies that advertise on social sites.

The study looked at nine categories that people tend to seek information about online on traditional Web sites to help with purchase decisions. They included travel or travel services, bank or financial services, clothes or shoes, eating out or restaurants, cell/mobile phones and service, personal-care products, cars or trucks, groceries or food, and prescription or OTC drugs. They found that respondents rarely go to social sites for purchase-related information.

For instance, the travel category had the highest percentage of respondents who regularly went to socnet sites for information, but that still was only 4 percent, with 24 percent saying they sometimes turn to social media for information. When it comes to prescription or OTC drugs, the number who regularly turn to socnets for info was just 1 percent, with only 10 percent using the sites occasionally for information.

If you’re trying to sell widgets via social networking sites, this information bodes ill. But perhaps you shouldn’t be trying to sell widgets through twitter anyway. Most people use those sites to post little comments, mostly about themselves or in response to someone else’s little comment. It is not a deep mode of communication, so for marketers to expect users to take the information on social media sites seriously is perhaps naive.

One reason the travel segment did a little better is because of the way travel writers have used socnets, especially twitter. Travel writers use their tweets to place enticing links to information about destinations, great fares or issues and events in the news. They realize that their information has value to people who travel, and they do a better job of using the socnets to brand themselves as experts in the travel field.

Internet users are trained to search for Web sites when they want information to influence a purchase decision. They see Web sites as a static repository of information and marketing communications. They see social media sites as primarily a two-way or more-way communications medium. They don’t expect twitter to have the kind of information they need to decide between a Ford and Chevy, or between Tylenol and Advil.

The limited marketing opportunities in the social media sites will be exploited by those who are savvy enough to provide a “What’s in it for me” call-to-action message in 140 characters or less.