Same with services. How many massages do you need? Well, if your insurance covers it, sometimes two a week. Even if you don't, some people will treat themselves to a massage every week. Mani/pedi? Same deal. Thing is, though, this business model is covered quite nicely by junk mail. As far as I'm concerned, Groupon - even more than Facebook - is an example of the dotcom bubble reinflating. This shit can't continue.
I had a friend that owned a bar and always had a $1 PBR on tap. The bar next to his charged 3$ a PBR. The bar next to him was always busier and the majority of the people at his competitors bar were drinking PBR's. Why didn't they come next door to him? Because his service sucked. Too many merchants are looking for a "silver bullet" to help drive revenue. It's no secret though, focus on your product and your service, throw in some targeted marketing and good things will happen. Groupon and FB pages for the most part don't do shit.