Being a private person i just shudder when hearing concepts about face recognition or auto tweets in this context.

^ "Have an alternate reality game with digital objects people can place/write on. Only Glass users can see the teddy bear in the corner, and only google glass users can see the giant graffiti wall where people sign their name."

I am sorry since for some this will not be considered as being a constructive contribution to the discussion, but there is an Anime out there about exactly(!) that. It's called Dennou Coil. Very good series. [Sorry, i just had to mention it! Forgive me.]