Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving an identifier associated with a user of a social networking service, identifying one or more first services provided by a first service provider, determining that the user is a user of the one or more first services based on the identifier, identifying first contacts of the user within each of the one or more first services, and generating a social vicinity index based on the first contacts, the social vicinity index including one or more potential contacts for the user within the social networking service.