As social media become a growing force at generating attention, marketers are increasingly turning to the less famous to help them pitch products. Auto makers and ad executives say tapping social-media stars can give a brand more credibility with younger shoppers than hiring high-priced celebrities. "People trust people like themselves, and when we can tap into these people, it will sound less like Ford tooting its own horn," says Scott Monty, Ford Motor Co.'s global digital-communications manager. Armed with images and video from their test drives, the stars of the Lexus campaign have been spreading the word by blogging, tweeting and posting images on Flickr.