16. The system of claim 15 , wherein each of the second user nodes are connected to the first node in the social graph.
16. The system of claim 15 , wherein each of the second user nodes are connected to the first node in the social graph. 17. The system of claim 16 , the processors being further operable when executing the instructions to: determine that a number of the second user nodes connected to the first node in the social graph is too small; and retrieve, from the social graph, information indicating that a social-networking account for each of one or more third users of the social-networking system has been compromised, wherein the third users are respectively associated with third user nodes in the social graph, wherein none of the third user nodes are connected to the first node in the social graph, and wherein the third users previously accessed the shared device; and wherein the message further comprises information indicating that the social-networking accounts for the third users has been compromised.