Not sure if something like this exist but what if you could use a system where people could subscribe to you (a bit like RSS) and get all your content (a bit like newsgroups, so that a third party would have a harder time to figure out at least some of the metadata) but the recipients could only decrypt what is addressed to them personally or to a group that they are part of...
SpiderOak's experimental Kloak provides such a system.
Encrypted pub/sub social will be the only way to ensure privacy, long-term. (Other than purely anonymous networks, which have other use cases.)