1. A method implemented by a computer system, the method comprising: generating, by the computer system, a set of content-based keywords based on content generated by users of a social network, wherein users are represented by nodes in a graph that represents the social network; labeling, by the computer system, nodes comprising the user nodes with advertising labels comprising content-based keywords from the set of content-based keywords that coincide with advertiser-selected keywords, wherein the advertiser-selected keywords are based on one or more terms specified by an advertiser; and outputting, by the computer system for each respective node, weights for the advertising labels which are determined from weights of the advertising labels associated with neighboring nodes that are related to the respective node by a relationship, each weight expressing a magnitude of a contribution of an associated advertising label to a characterization of the respective node.