The increasing participation of people in online activities in recent years like content publishing, and having different kinds of relationships and interactions, along with the emergence of online social networks and people's extensive tendency toward them, have resulted in generation and availability of a huge amount of valuable information that has never been available before, and have introduced some new, attractive, varied, and useful research areas to researchers. In this paper we try to review some of the accomplished research on information of SNSs (Social Network Sites), and introduce some of the attractive applications that analyzing this information has. This will lead to the introduction of some new research areas to researchers. By reviewing the research in this area we will present a categorization of research topics about online social networks. This categorization includes seventeen research subtopics or subareas that will be introduced along with some of the accomplished research in these subareas. According to the consequences (slight, significant, and sometimes catastrophic) that revelation of personal and private information has, a research area that researchers have vastly investigated is privacy in online social networks. After an overview on different research subareas of SNSs, we will get more focused on the subarea of privacy protection in social networks, and introduce different aspects of it along with a categorization of these aspects.