In one embodiment, a method includes receiving data uniquely identifying a particular user to the verification authority and a request to access a shared device. The shared device being configured for use by at least a number of users. The method also includes accessing a social graph of the particular user to determine whether one or more users in the social graph have previously accessed the shared device; and displaying on a display of the mobile device information indicating which of the users in the social graph have previously accessed the shared device.