The smart phone, like a small computer, providing not only the functions of normal phones and wireless Internet access, but also all kinds of application tools, serves as a platform of the sources of real-time information, information sharing and information exchange in our daily lives. As a result, the extract the crucial digital evidence in the smart phone, it has now become a challenge in the technology time. In particular, it is for when the smart phone applications are committed as the illegal contacts. In this paper, we discuss the most five popular mobile social networking applications in iPhone usages. The backup files of social networking applications could offer us the crucial evidence in this paper study. The relative information of social networking applications operated in iPhone is able to be accessed if necessary, even though iPhone has been destructed or encrypted by the suspect. It could help the investigators to exactly reconstruct the crime venue and find the truth.