Social bookmarking services allow a user to make her personal collection of favorite web resources accessible by the public. The content of this collection can attract users of "similar minds" and therefore has tremendous potential to enable networking and collaboration. In this research, we analyzed a large dataset collected from one of the most popular social bookmarking services. To understand why there is a large gap between a user's explicit network and her implicit user-user association networks based on common resources or common tags, we compared a users' bookmark resources and tags to those of her explicit network members. Our results suggest that a typical social bookmarking service user does not create her explicit network based on common interests. We discuss the implications behind the gap between a user's explicit network and implicit network and propose solutions to enhance and improve the "social" functions of social bookmarking services.