Social networking sites (SNSs) have gained immense popularity. This phenomenon has created new marketing opportunities for companies to leverage on online networks. In particular, SNSs allow the conceptualization of a new type of product endorser: the “friend.” However, the effectiveness of friends as endorsers on SNS remains unknown compared with other types of product endorsers. This paper investigates the impact of three critical factors on advertising effectiveness in SNS: tie strength, endorser expertise, and product type. Using a 2 × 2 × 2 experimental design, we found interaction effects among these factors. For hedonic products, strong-tie endorsers were more effective than weak-tie endorsers in influencing purchase intention. However, for utilitarian products, high-expertise endorsers were more effective than low-expertise endorsers. Based on these results, we discuss key theoretical and practical implications for research on SNS advertising.