Well, what if - and I'm stretching here - you as the seller could make deals that were only found/purchasable within Facebook? If Pampers gave a $5 coupon only to FB users, that would make it work, wouldn't it? In other words, if you went to amazon.com and browsed Pampers, you would not get that same discount
Why would Amazon encourage me to shop on Facebook instead of Amazon, though?
I'm not sure that Amazon would but maybe the companies that develop the products might have a promo for building their Facebook page? I don't think it's really all that hard to think of a reason.