Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, including actions of, for each user of a plurality of users of a social networking service, determining a user activity score, the user activity score being based on activity of each user within the social networking service, determining that a first user activity score of a first user is greater than or equal to a first threshold user activity score and that a second user activity score of a second user is less than a second threshold user activity score, and, in response to the first user activity score being greater than or equal to the first threshold user activity score and the second user activity score being less than the second threshold user activity score, transmitting instructions to display a prompt to the first user to encourage the first user to interact with the second user.