Drawing on the uncanny valley effects (UVE) hypothesis and related works on avatar-based interactions, this study investigates the role of UVE (i.e., a feeling of eeriness), induced by avatar realism and animacy, on the perceived trustworthiness (PT) of an avatar user, and subsequent decisions to befriend that avatar user in a virtual social networking service (SNS). Specifically, the current study examines whether (1) the realism (hyperrealistic vs. cartoonish) and animacy (animate vs. still) of avatars, rendered through a 3D scanning technology, will induce a feeling of eeriness as a result of UVE; (2) the degree to which perceivers experience a feeling of eeriness will negatively bias the PT of an avatar user; and (3) PT, determined by UVE, will serve a role as a gating mechanism for the decision whether to befriend the unknown avatar user in a virtual SNS. Results from a two (realism: cartoonish vs. hyperrealistic) × two (animacy: still vs. animate) between-subjects online experiment (N = 134) indicated that a feeling of eeriness, induced by the realism and animacy of avatars, negatively biased the PT of the avatar user, and this, in turn, led avatar perceivers to reject a friend request sent from the unknown avatar user. These results call into the question the effectiveness of rendering avatar realism in social virtual environments. Overall, the findings of our research provide theoretical as well as practical implications for the design of avatars in virtual SNSs.