This is an interesting idea that uses email/IMAP as a transport for social message passing. It plays to the strengths of multipart email messages having a human-readable part that is as rich as possible for email client viewing (when users aren't in a social-aware application), and a machine readable part (JSON) for plugging in to the rest of the social infrastructure.

Here is their own summary of problems with this approach (from the article):

  * Not all users have a suitable email service (requires IMAP to support the
    “read it if you feel like it” model).

  * Servers tamper with messages as a normal part of existing infrastructure
    (especially mailing lists and spam filters).

  * Basic IMAP does not provide the full gamut of features required (some mail
    servers don't support all IMAP verbs).

  * Mail systems may have standard protocols, but individual implementations
    may have different performance characteristics.