Obviously, the community structure in the abovementioned research provides insights for a more accuraterecommendation or link prediction. However, some of theworks use an existing network to detect the communitystructure within. And some of the methods requirecertain extra social information like trust, interactionsbetween users or friendship to build a user-user network.Some of the research evaluates user trust relationsbased on ratings or similarity, and performs communitydetection on the user-user trust network.