Groupon is primarily good for discovery, but it's up to merchants to capitalize on referrals from their Groupon customers. It works particularly well for small operators who need to put a quirky business on a map.<p>For example, a friend who lives in San Francisco bought a Segway tour of the city, which she would never on earth do had she not seen it on Groupon. She then asked 50 people to join her tour, and then plastered the photos all over Facebook. A lot of people now know these tours exist, and next time we have friends visiting from out of town, we'll be asking her about it. If the Segway tour operator had offered her a promo for referrals, all the more reason for us to remember the tours and book, preferably via her referral link.<p>I suspect the effect is similar to getting your startup covered in Techcrunch. Suddenly you get tons of pageviews and signups, but it's up to the startup to keep people coming back.