So far the Internet have been developing that gradually aggregated different social interest communities in which provide various content respectively. Among these social communities intensely share and swap information that has formed different social networked resources. For the sake of keeping the goals of establishing public libraries, they should get aware of various virtual communities of which characteristics and inter relationships. Therefore, we can integrate the synthetic effects of the new type of Internet resources into public libraries operation. This paper aims to introduce the concepts and forms of social network, popular social network software, in addition to discuss user's behaviors of social network software and relationships among members of different communities to assist public libraries to plan their social network services from the perspective of public libraries.