A computing device comprising: a storage containing: a contact entry constituting information about a user of an address book service, the contact entry including a first human-readable string associated with both of first fields and first values, and a second human-readable string associated with both of second fields and second values that is different from the first human-readable string for filtering the contact entry into a first identity constituted by the first values, and a second identity constituted by the second values; a plurality of other contact entries for individual contacts of the user, the contacts being classified under different categories; and a processor configured to execute an address book server of the address book service to: receive a request for the contact entry from a requestor, the requestor being a contact of the user and having a contact entry; and transmit to the requestor either the first identity, or the second identity, according to a rule wherein the rule causes the first identity to be provided when the category classification of the requestor is determined to belong to a first category related to the first human-readable string, and wherein the rule causes the second identity to be provided when the requestor is determined to belong to a second category related to the second human-readable string; wherein the rule is defined by the user a storage containing: a contact entry constituting information about a user of an address book service, the contact entry including a first human-readable string associated with both of first fields and first values, and a second human-readable string associated with both of second fields and second values that is different from the first human-readable string for filtering the contact entry into a first identity constituted by the first values, and a second identity constituted by the second values; a plurality of other contact entries for individual contacts of the user, the contacts being classified under different categories; and a contact entry constituting information about a user of an address book service, the contact entry including a first human-readable string associated with both of first fields and first values, and a second human-readable string associated with both of second fields and second values that is different from the first human-readable string for filtering the contact entry into a first identity constituted by the first values, and a second identity constituted by the second values; a plurality of other contact entries for individual contacts of the user, the contacts being classified under different categories; a processor configured to execute an address book server of the address book service to: receive a request for the contact entry from a requestor, the requestor being a contact of the user and having a contact entry; and transmit to the requestor either the first identity, or the second identity, according to a rule wherein the rule causes the first identity to be provided when the category classification of the requestor is determined to belong to a first category related to the first human-readable string, and wherein the rule causes the second identity to be provided when the requestor is determined to belong to a second category related to the second human-readable string; receive a request for the contact entry from a requestor, the requestor being a contact of the user and having a contact entry; and transmit to the requestor either the first identity, or the second identity, according to a rule wherein the rule causes the first identity to be provided when the category classification of the requestor is determined to belong to a first category related to the first human-readable string, and wherein the rule causes the second identity to be provided when the requestor is determined to belong to a second category related to the second human-readable string; wherein the rule is defined by the user