Groupon founder Andrew Mason has built a castle. His Internet coupon empire will harvest some $2.4 billion in sales this year thanks to rapid growth in its wittily worded email offers for discount pole-dancing lessons and two-for-one chicken Parmesans. Mason’s next trick needs to be digging a moat around his business. That’s arguably tougher, and it means being more like online restaurant-booking outfit OpenTable—or buying it.

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