Maybe we won't see many coupons for LBS until they have a large enough slice of the retailer's demographic (or just a large enough % of the population) and few people will sign up to use these services if they don't have these coupons.
Facebook on the other hand is a different beast. A massively larger user base and greater brand recognition with companies. How many ads do you see on TV where the URL on the screen is to facebook.com/some-household-brand. Businesses are already savvy to the fact that a great proportion of their customer's traffic is on Facebook. Seems like a small jump to move your coupons from your Facebook page to your location/"Place" on Facebook. Gowalla and Foursquare don't have their foot in the door like Facebook does.