Looks like this should work in store too, not just at Costco.com:
How it works:
American Express takes what you "like" on Facebook and gives you offers you may love.
Sync your Card to the program
Select the offers you love by clicking "Get Offer" and following instructions
Use your synced American Express Card in-store or online and receive a discount or have statement credits issued directly to your Card account depending on the offer.