1. A computer-implemented method comprising: determining, with a computer server system that includes one or more processors, initial label values for at least a portion of a group of users of a social network, wherein each of the label values indicates a probability that a user is interested in a subject indicated by a label; associating a first user with a first plurality of users from the group of users and a second user with a second plurality of users from the group of users based on social relationships associated with the first user and the second user, respectively, wherein the first plurality of users includes the second user and the second plurality of users includes the first user; determining label values for the first user and the second user based on bi-directional propagation of label values, wherein the bi-directional propagation of label values comprises: determining a first label value for the first user based on a combination of label values for the first plurality of users including a label value for the second user; and determining a second label value for the second user based on a combination of label values for the second plurality of users including the determined first label value for the first user; and outputting the first label value for the first user and the second label value for the second user.