1. A system for evaluating content items served by the system to multiple users who initiate sharing events regarding the content items, the system comprising: a processor configured to: generate a centrality matrix M based on sharing event data reflecting the sharing events initiated by the multiple users, wherein: the matrix M comprises a number of rows and a number of columns equal to the number of users in the multiple users; each row corresponds to a destination user in the multiple users; each column corresponds to a source user in the multiple users; each element in the matrix has a corresponding destination user and a corresponding source user, and contains either a zero value or a non-zero numerical value; each zero value in the matrix indicates that for none of the content items did the corresponding destination user initiate a sharing event before the corresponding source user initiated a sharing event for the same content item; each non-zero value in the matrix indicates that a sharing event regarding at least one content item was initiated by the corresponding destination user before a sharing event was initiated by the corresponding source user for the same content item; and all values in each column sum to the value of one; compute an eigenvector d such that Md=M; assign the value of each index of eigenvector d as a user reputation score for a corresponding user among the multiple users; and for each content item, combine the reputation scores of users that initiated sharing events regarding the content item, to produce an item reputation score for the content item; and a memory coupled to the processor and configured to provide the processor with instructions.