Although privacy or scoping audience is more often than not controlled by a service provider through access controls, it may also be of some use to define how to indicate the intended audience (or privacy) rules for a given entry. Specifically, a user needs be in the position of acting as policy decision point (PDPs) in order to indicate to their privacy settings to policy enforcement points (PEPs).

Note: There are several use cases that have come up in our OpenSocial work. Here's the page that describes the interest lists, which are really the similar concepts.

Use cases

• Directing an activity to a subset of one's contacts
• Posting an activity publicly, but notifying certain contacts of the new post (similar to email's carbon-copy (cc:) feature).
• Sharing content with a specific person
• Reposting an activity with an expanded audience 

Lists & Groups

Individual users may maintain their own lists of contacts — often called "groups" in Address Book software — for managing access, or merely for convenience. These lists can provide a convenient mechanism for grouping contacts together as audiences. 

For example, I may have lists of coworkers or college friends that I maintain, and use those lists to gate access to photos that I upload.

Groups, however, are typically represented as shared spaces — where different roles are taken by group members. Yet groups in this sense still represent a list of people, and could therefore also be used as a target audience of an activity.

ActivityStreams may benefit from expressing an audience or audiences directly in the format to enable client applications to reflect an actor's preferred audience for the actions they engage in.

Examples

FriendFeed

Google Buzz

Facebook

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