Last week, Brafton reported that Facebook is developing new (potentially local) ad units, and now the company has unveiled Sponsored Stories. For brands looking to get their products and services recommended to users by friends, this ad format could go a long way in generating word-of-web referrals.

The Sponsored Stories are user-generated promotional content. Facebook product marketing head Jim Squires told Mashable that these ads are "a way for marketers to sponsor activities that happen throughout the News Feed."

Brands can turn Facebook users' Likes, page posts, check-ins and actions with companies' custom applications into feature stories that will appear in the right-side column of friends' Facebook homepages. Although advertisers have no control over how these "stories" turn out, positive social mentions from trusted friends could be key to converting Facebook users.

As Brafton has reported, teens and parents turn to social sites and solicit friends’ opinions when making purchase decisions. This means user-generated social content can be an invaluable promotional tool.

Mashable reports that several major brands, including Coke, Anheuser Busch and Levi's, are already on board. Other brands may soon be following suit. Sponsored Stories adds to the growing number of Facebook ad options, and Mark Zuckerberg's social network is expected to drive overall social ad spend beyond $3 billion.