This summer we have seen plenty of campaigns with QR codes. Recent one is from Diesel: you visit a Diesel's store, scan QR code next to an item you like, and click on like button on the mobile Facebook page. That's pretty much it. Check this video:

I would rather make it funny and make QR code link lead to status change that's relevant to an item. E.g., if you scan QR code next to a red sweater your Facebook status changes to "Now thinking about how fantastic I'll look in this red one on the autumn streets."