22. A computer system for sharing content items among a plurality of users, the system comprising: a user data store configured to store a user record for each of the plurality of users, wherein the user record for each user comprises one or more user tokens associated with that user, each user token representing a visibility policy under which that user is granted access to content items posted by one of the plurality of users; a content data store configured to store an item record for each of the shared content items, wherein each item record comprises one or more content tokens, each content token representing a visibility policy for determining which of the plurality of users are granted access to content items posted by one of the plurality of users; posting control logic configured to receive a visibility instruction for a first one of the shared content items from a first one of the plurality of users, the visibility instruction indicating that the first content item should be shared according to a first visibility policy, and to store a first content token in the item record for the first shared content item, the first content token representing the first visibility policy; and access control logic configured to receive a request from a requesting one of the plurality of users to access the first shared content item and to grant access in the event that one of the user tokens in the user record for the requesting user matches a content token associated with the first content item.
22. A computer system for sharing content items among a plurality of users, the system comprising: a user data store configured to store a user record for each of the plurality of users, wherein the user record for each user comprises one or more user tokens associated with that user, each user token representing a visibility policy under which that user is granted access to content items posted by one of the plurality of users; a content data store configured to store an item record for each of the shared content items, wherein each item record comprises one or more content tokens, each content token representing a visibility policy for determining which of the plurality of users are granted access to content items posted by one of the plurality of users; posting control logic configured to receive a visibility instruction for a first one of the shared content items from a first one of the plurality of users, the visibility instruction indicating that the first content item should be shared according to a first visibility policy, and to store a first content token in the item record for the first shared content item, the first content token representing the first visibility policy; and access control logic configured to receive a request from a requesting one of the plurality of users to access the first shared content item and to grant access in the event that one of the user tokens in the user record for the requesting user matches a content token associated with the first content item. 23. The computer system of claim 22 , further comprising: token propagation control logic configured to automatically determine which user records should include each user token based on the visibility policy and the trust network data and to automatically add user tokens to user records based on the automatic determination; and wherein the user record for each user further comprises trust network data identifying one or more other users as being friends of that user and wherein each user token represents a visibility policy that is defined by reference to a maximum degree of separation in the trust network from a posting user.