Shake Shack's free burger giveaway won't help boost diminishing sales

Shake Shack celebrates 100th restaurant with free burgers

What's better than a cheeseburger? A free one!

Shake Shack is celebrating the opening of its 100th restaurant by giving free burgers to the first 100 guests at all its locations worldwide (except those in ballparks and stadiums) starting at 10:30 a.m. Tues., Aug. 16.

"From our first-ever Shack in Madison Square Park twelve years ago, to so many incredible places in between, opening our 100th Shack at Boston Seaport is an epic milestone for us," Randy Garutti, CEO of Shake Shack, said in a statement.

As if free burgers weren't enough, the company is launching a limited-time only burger in all five of its Massachusetts locations between Aug. 16 and Aug. 21.

The Coppa Burger, created by Jamie Bissonnette and Ken Oringer, owners of Coppa, an award-winning Italian restaurant in Boston, features a beef patty topped with provolone cheese, griddled mortadella, cherry peppers, caramelized onions, mayo and shredded lettuce.

"We approached the burger with the flavors we love — a classic Italian American grinder — in mind," Bissonnette said in a statement. "Shake Shack burgers offer a great foundation to create something killer!"

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Shake Shack, which opened its first store in 2004, could use a boost to foot traffic. The company posted weaker-than-expected same-store sales on Thursday, sending the stock tumbling. The burger chain's stock is down more than 40 percent in the last year amid growth and valuation concerns.

While the promotion has sparked some buzz on social media, it might be a short-lived boost.

The promotion will have "very limited impact," said Darren Tristano, president of research and consulting firm Technomic.

"It will give them a small lift but likely won't impact the soft sales the we [are] seeing today," he told CNBC.

Tristano noted that the customer cutback is likely the result of "election jitters." And he's not alone in subscribing to this theory. McDonald's, Wendy's and Ruby Tuesday also blamed the presidential campaign for poor restaurant sales.

In addition, restaurants are facing headwinds from grocery chains, convenience stores and meal delivery companies, who are aggressively competing for a share of consumers' stomachs.