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: receive sharing event data reflecting the sharing events initiated by the multiple users, including, for each content item, an initial sharing event initiated by an initial user; determine, for each content item, a time-based order of the sharing events reflected in the sharing event data; generate a graph based on the sharing event data, the graph comprising: a separate node corresponding to each of the multiple users that initiated a sharing event; and for each content item, for each given user that initiated a sharing event for the content item, other than the initial user who initiated the initial sharing event for the content item, at least one directed link from the given user's corresponding node to a node corresponding to another user who initiated a sharing event for the content item before the given user; determine user reputation scores for the multiple users based on connectivity of the graph; and for each content item, combine the user reputation scores of the users that initiated sharing events for the content item to produce a score for the item; and a memory coupled to the processor and configured to provide the processor with instructions.