CHICAGO/SAN FRANCISCO (Reuters) - Groupon and its compatriots in the much-hyped daily deals business were supposed to change the very nature of small-business advertising. Instead, it is the daily deal vendors that are racing to change as evidence mounts that their business model is fundamentally flawed. Groupon last week reported another quarter of disappointing earnings as its core business stagnated, sending its stock down 30 percent to an all-time low of $2.76. Its biggest rival, Living Social, is piling up losses, and part-owner Amazon. ...

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