Pretty much the same time that Facebook launched its "Groupon Clone" they are calling "Facebook Deals", Google is launching "Google Offers" in 4 cities (New York City, Oakland, Portland Oregon, and San Francisco). Will all of these "Deal of the Day" websites end up cannibalizing each other, or will they all live in harmony? My all-time favorite has been Woot.com which was the first site that I saw with a one deal only for today type of concept. To a lesser extent I have always enjoyed various coupon websites which help to enhance my ecommerce experience on websites I am making my purchases on (I usually search for coupons before finalizing an online purchase).
For Google as well as Facebook, they have a clear advantage over any of the other deal sites because they can integrate their other properties (Adsense, Google Maps, etc.) into this new program.