google is in an excellent position for this: they "only" have to assign a keypair to every profile. there's a small problem in that the private key would have to be stored in their cloud for good usability, but it's otherwise possible.
Re:Google+ and Privacy: A Roundup
by mike-cardwell on Monday July 04, 2011 @06:40AM
(#2726126)
They're not going to do that though because of their commercial interests. And a key stored by the service provider is equivalent to a service provider storing the data unencrypted and promising to not look at it. Just like Dropbox.
Until we have a social network where every message posted is only viewable by the sender and the intended recipients, there will always be room for improvement.