6. The method of claim 1 further comprising: defining a second user token and a second content token, the second user token and the second content token each representing a second visibility policy for determining whether a particular one of the plurality of users is granted access to content items posted by the first user, wherein the second user token and the second content token are matching tokens; and associating the second user token with each user who is granted access to content items according to the second visibility policy.
6. The method of claim 1 further comprising: defining a second user token and a second content token, the second user token and the second content token each representing a second visibility policy for determining whether a particular one of the plurality of users is granted access to content items posted by the first user, wherein the second user token and the second content token are matching tokens; and associating the second user token with each user who is granted access to content items according to the second visibility policy. 8. The method of claim 6 further comprising: receiving a visibility instruction for a second content item from the first user, the second visibility instruction indicating that the second content item should be shared according to the second visibility policy; and in response to the visibility instruction for the second content item, associating the second content token with the second content item in the index of content items, wherein when one of the plurality of users requests access to the second content item, the request is granted or denied based on whether the second content token matches a user token associated with the user.