Systems and methods to define a social network for a user using data implied by communications of the user with others. In one embodiment, a method includes scanning a set of messages (e.g., emails to and from a user of a client device) to identify a plurality of primary persons addressed in the messages as a primary recipient and to identify a plurality of secondary persons addressed in the messages as a secondary recipient; mapping the primary persons to a plurality of friends and mapping the secondary persons to a plurality of other persons; and defining a social network model based on the friends and other persons, wherein the model comprises a friends category including the friends and an all category including the friends and other persons.