A method performed by one or more processing devices includes transmitting, to a first client device, content items, wherein one of the content items is selected by a user of the first client device; receiving, from a second client device, information indicative of a selection, by a user of the second client device, of a reference in a social networking system to a resource associated with the selected content item; wherein the user of the first client device has a social connection in the social networking system to the user of the second client device; and generating, based on the information received, one or more viral metrics for the selected content item.