What customer wouldn't want to score a deep discount on dinner, beauty treatments and other services, especially during a downturn? Barely three years old as an industry, online group buying sites are witnessing rapid growth, as more subscribers sign up, more partner businesses sign on, revenues climb and venture capitalists swarm to invest, further driving up business valuations as a result. The most prominent group buying site, Chicago-based Groupon, has 2011 revenues estimated at between $3 billion and $4 billion. Google last December offered to buy the firm for $6.4 billion. After the acquisition was unsuccessful, the search giant launched its own venture, Google Offers. Facebook, too, is entering the space, joining the roughly 500 group buying sites that have emerged worldwide. But much of that "wild exuberance" is miscalculated and could bring ruin to investors, warns Wharton marketing professor David Reibstein in an interview with Knowledge@Wharton. Taking Groupon as a case in point, he says the industry's current growth rates are unsustainable. Also, he faults the site's business model, arguing that it will leave customers, suppliers and investors disenchanted. Get the full story at Knowledge@Wharton