Retailers routinely post customers’ product reviews online, hoping that favorable comments will boost sales. But there’s a more powerful influence on shoppers that retailers have yet to harness: the advice of friends.

Many retail sites have email-a-friend features, which make it easy to ask friends what they think of a product by sending them a link to the page it’s on. But that approach has one big drawback: Shoppers are unlikely to get immediate feedback while they’re still at a retailer’s site, so their decisions may be delayed, putting sales at risk. Some retailers have also tried unsuccessfully to launch their own social networks.

Now, retailers are exploring ways to link their sites to social-networking sites like Facebook, MySpace and Twitter, among others. The biggest networking sites have developed tools that make it possible for a member shopping on a retailer’s site to get immediate feedback in the form of any reviews friends have left there, as well as a history of friends’ purchases on the site.

Meanwhile, networking sites and third parties have created tools that allow shoppers on a retailer’s site to post information and opinions about products on a number of social networks, or to chat on the retailer’s site with any friends who happen to be available. And some retailers are making software code available that anyone can use to develop applications that link the retailers’ sites to social networks.

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