Heidi Steffen and her husband used to treat themselves most weeks to steak at Sodak Shores, a restaurant overlooking a lake near their hometown of Milbank, S.D. Then they each got an iPhone, and the rib-eyes started making fewer appearances.

"Every weekend, we'd do something," said Ms. Steffen, a registered nurse whose husband works at a tire shop. "Now maybe once every month or two, we get out."

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