- you can do 'retweets' via RSS,
- you can do 'only see who i follow'
- you can do 'only publish to some people' (subscriber-only feeds, though they are hacks)
- you can share arbitrary length things and arbitrary filetypes through rss
- the posts are yours because they live on your blog or whatever
you just need a contacts list that lets the magic happen. I don't have the cycles to go deep on it.
a lot of people derisively say "you are describing RSS"
instead I'm pointing at inspiration