The useful part of block chain is the Merkel tree, where each small piece of data is tamper proof and consistent because it hashes the data that came before it.

The useful part of block chain is the Merkel tree, where each small piece of data is tamper proof and consistent because it hashes the data that came before it. Having all the social data on one big chain would be silly, because it would never scale. Scuttlebutt is a protocol just like you describe and is already being used by different clients and plugins to build a distributed social network that works quite well.