This might end of being very complicated, and comes in two parts.
Create a alternate lastn view, that not only shows all of the entries I have posted in my journal, but also all of the entries I have posted in any shared journals or communities.
When a user adds a friend, allow them to add that person with an optional "Shared Posts flag" meaning that whenever this user posts anywhere (personal journal, shared journal, or community) it will show up on that users friends page.
Just a thought. Comments?