Visisble to Friends?

I’m thinking of making a variation on the social networking thing in a WordPress hack.
This is how I imagine it going:
In your user profile, you can identify another user as a friend. Then, when you post on the blog, you can label a post as “only viewable by friends” and it will then be invisble to everyone else. You could also make a post visible to “friends of friends” and so on. And you would also have a preference as to who you would ignore, as in you could choose to ignore anyone who is more than three friends seperated from you, for example.
So, ultimately, you could have a hundred people posting to the blog, but everyone only sees the posts from their own circle of friends.
Has anyone done a similar hack, yet?