The newest version of the Postagram app adds a QR code to all post cards sent out, which — when scanned by a QR reader that was just added to the app — lets recipients download a digital copy of the photo and share it on Twitter. It also lets the sender of the post card know it was received — success!!! — and automatically adds the senders’ address to the recipients’ address book so they can send their own Postagrams in reply.

Source: gigaom.com