1. A system for evaluating a subject content item based on user reputation information, comprising: a processor configured to: receive sharing event data reflecting sharing events initiated by multiple users for multiple items, including the subject item; materialize a graph from the received sharing event data, wherein: each node of the graph represents one of the multiple users; and a given node representing a given user who initiated sharing events for one or more items among the multiple items is linked only to nodes representing other users who initiated sharing events for the one or more items earlier in time than the given user; determine user reputation scores for the multiple users based at least in part on the graph, without receiving any explicit ratings from any of the multiple users; and assign a score to the subject item based at least in part on the user reputation scores of the multiple users; and a memory coupled to the processor and configured to provide the processor with instructions; wherein each sharing event is one of: a digg; a bury; a contribution of an item; a comment on an item; and activation of a link to share an item via a social network service.