Forgive the rambling, this is the first time I've written any of this down...
My idea is to use email as a transport for 'social attachments' that would be read using a custom mail client (it remains to be seen if it should be your regular email client or have it be just your 'social mail' client. But... if using another client as regular email, users would have to ignore or filter out social mails). It could also be done as a mimetype handler/viewer for social attachments.
Advantages of using email: - Decentralized (can move providers) - email address as rendezvous point (simple for users to grasp) - Works behind firewalls - Can work with local (ie Maildir) or remote (imap) mailstores. If using imap, helps to address the multiple devices issue. Could also use replication to handle it too (Syncthing, dropbox, etc)
Scuttlebutt looks like a nice alternative though. Will be following closely.