In one embodiment, a method comprising by one or more computing devices, transmitting, from a first mobile device, a first message that includes a first user identifier corresponding to a first user of the first mobile device, wherein the first user identifier corresponds to a user account of the first user in a social-networking system, receiving, at the first mobile device, one or more second messages that includes a second user identifier corresponding to a second mobile device within a direct radio connectivity of the first mobile device, wherein the second user identifier corresponds to a respective user account of a second user in the social-networking system, and storing, at the first mobile device, information received in the second message for access by one or more applications hosted on the first mobile device.