Foursquare created the concept of "checking in" to a venue/shop - checkin often enough and you can become the "mayor" and perhaps receive a reward from the establishment (if they are clued up enough to see the benefit!)
Checking in could mean you receive special discounts or, perhaps, receive the modern equivalent of "Green Shield stamps" which can be redeemed at other participating venues.

However, I think there is also mileage in being able to "auto checkin" - I.e. As you wander from shop to shop the (e.g.) Foursquare database automatically records the event - the benefit here is that you seemlessly intereact with the rewards program.

This will mean that our Twitter/Foursquare account will be something which is discussed when paying for items at the till "have you got a twitter account sir?" - hand it over and receive the relevant reward - it's not as if they can spam you! Yes there are privacy issues - tight controls will need to be put in place as to who sees what and we'd also need fine grained control over things like "auto checkin" but these are solveable problems.

Oh yeah - and I'm positive Facebook will move into this arena!

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