1. A method comprising, by one or more computing devices: receiving identifier-information from a client device of a first user, wherein the identifier-information is associated with a source of an incoming communication made to the client device of the first user; accessing a social graph of an online social network, the social graph comprising a plurality of nodes and a plurality of edges connecting the nodes, each of the edges between two of the nodes representing a single degree of separation between them, the nodes comprising: a first node corresponding to the first user, and a plurality of second nodes that each correspond to a second user; determining whether one or more reporting users have flagged the source of the incoming communication as a spam source, wherein the reporting users are second users corresponding to one or more second nodes that are within a specified degree of separation from the first node; and sending, to the client device of the first user, a spam-notification object comprising information that identifies one or more of the reporting users.