Mixi is the most popular SNS in Japan.
It is a centralized system, which from my point of view is not desirable.

It’s functions are as follows:

I. Information Publishing
(1) Profile page : Displayed items change depending on
whether the viewer is in the friend list.
(2) Diary : Access controlled blog. ACL is on the
friends, fof, general public.
(3) Review : Use Amazon Web Service to write
reviews on books etc.
(4) Message: Mail like function intended just inside
mixi.
(5) Photo Album : ACL capable.
(6) Invitation：Invite friend to the network.

II. Information Receipt
(1) Search (Friend, Community)
(2) Show Profile
(3) Friends’ Diary : Aggregate the friends’
diary.
(4) Friends’ Review：Aame as above, but for
reviews.

Now, how to achieve it on the distributed system.

I(1): Simply change the items to be displayed
according to the result of the authentication.
Just a SAML enabled web page will do.
I(2): It is nothing but a SAML authenticated blog.
RSS access is also controlled.
I(3): SAML enabled Amazon web service client.
I(4): Something like i-name contact service.
I(5): Same as I(1)
I(6): Authorization request based on SAML Assertion.

These simply mean that registration and authorization
enabled page based on SAML Assertion will do.

II(1): Google, or some kind of registry.
If it were i-name, i-name whois will do.
II(2): Aka Viewing homepages.
II(3)(4): RSS aggregation.

Well, how simple. No reason to be stuck with
a proprietary centralized system. I(4) perhaps
calls for a standerdized protocol, though.