Social network sites (SNSs) can create new marketing opportunities by leveraging on members’ online social networks. In particular, such sites can produce a new type of product endorser – “friend endorser”, whose effectiveness remains unknown as compared to other types of product endorsers. This paper investigated the impact of three critical factors on advertising effectiveness of different types of product endorser on SNSs - tie strength, endorser expertise, and product type. A 2 × 2 × 2 factorial design was used to test the interaction effects among the three variables. The results indicate that, for hedonic products endorsed on SNSs, strong-tie endorsers are more effective than weak-tie endorsers, regardless of their expertise on the endorsed products; but for utilitarian products endorsed on SNSs, high-expertise endorsers, as compared to low-expertise endorsers, result in higher consumers’ purchase intention, regardless of their tie strength with consumers. Based on these results, theoretical and practical implications were discussed, and suggestions for future research in social network advertising were also provided.