1. A computer-implemented method comprising: obtaining, by a computer system, information that identifies interactions among users of a social network; generating, by the computer system, a graph that is based at least in part on the obtained information and comprises i) nodes that represent the users of the social network and ii) edges that connect the nodes and that represent relationships between the users; assigning, to at least a portion of the nodes in the graph and for one or more labels, initial label values that indicate levels of interest of users associated with the portion of the nodes in content associated with the one or more labels; determining, for the nodes in the graph, label values for the one or more labels based on iterative propagation of the initial label values among the nodes using the edges of the graph, wherein iterative propagation comprises, for a particular node from the nodes in the graph, determining particular label values for the particular node at each of a plurality of iterations by combining, at each of the plurality of iterations, neighboring label values for neighboring nodes that are connected to the particular node by a portion of the edges of the graph; and identifying, by the computer system for a particular label from the one or more labels, one or more users to provide with particular content that is associated with the particular label, wherein the one or more users are identified based on the determined label values for the particular label.