Social network is a social structure which is consisting of social groups and links between these groups. A social network can be depicted as a graph where the nodes represent the groups or the participants, such as people or organizations. Due to rapid development of Internet, communication and collaboration between people has become easier. In recent years, digital social networks such as Facebook, Twitter and Weibo have become an important part of our daily lives. These platforms allow us to exchange large amount of information. Given the enormous amounts of data, social networks have direct reflection to human society. However, mining and analyzing social network data is a non-trivial task, which faces two challenges: imperfection and dynamism. First, all the received data are incomplete and secondly all social networks are particularly dynamic. Forecasting, for lost or unconnected links to current social networks that have recently been added or deleted, is very important for future social networks. This problem is commonly known as link prediction, and over the past decade, several efforts have been made by psychologists, computer scientists, physicists and economists to resolve this problem.