BACK IN FEBRUARY 2006, when a Yelp member named Brenna F. reviewed the Seattle restaurant Machiavelli, nobody thought twice about it. And if someone did, we doubt they imagined that it would have any impact on chain restaurants like McDonald’s or Applebee’s. Brenna gave Machiavelli three stars (out of five) and wrote: “Good pasta, reasonable prices, and cozy seating. Go here with friends, but not with a first date.”

Ristorante Machiavelli is one of more than a thousand restaurants in the city of Seattle. You’re unlikely to read about it in Gourmet magazine, and it doesn’t have the advertising budget of a chain such as Olive Garden. But Yelp helped Machiavelli (and other small restaurants) gain something that, up until ...

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