FORTUNE — Ben Silbermann can’t stop staring at the refrigerators. The Pinterest co-founder and CEO and I are standing in the break room of his company’s garage-size Palo Alto office. He’s just flown back from Austin’s SXSW interactive festival, and a redesign of his website is two days away. It’s all a little overwhelming. But at this moment his full attention is focused on three glowing refrigerators. Sometime during his brief absence, a service has delivered them fully stocked and branded with the company logo. They’re wedged into the tiny backroom behind the foosball table that three employees — roughly 15% of his workforce — are using for a conference. “I’ve been gone for one day, and it’s so upscale,” he says. “We used to just run to Costco all the time.”
That was before. Before Mitt Romney’s wife, Ann, began organizing family photos on Pinterest. Before Reese Witherspoon gushed to Conan O’Brien that it was “a collection of the most amazing, wonderful craftiness on the earth!” Before the U.S. Army issued a guide for how to use it, and before Pinterest emerged as the fastest-growing website of all time. In March the site registered 17.8 million users, according to Comscore, a 52% jump in just one month — and it isn’t even open to everyone (would-be “pinners” must still request an invitation to join).
Pinterest, for the uninitiated, is a deceptively simple-sounding, insanely addictive social media site that lets users collect and share images on digital pinboards. Most social-networking sites have first become popular among tech’s early adopters along the country’s coasts. But Pinterest found its most passionate users among the Midwestern scrapbooking set — a mostly female group — who have turned to it to plan weddings, save recipes, and post ideas for kitchen renovations.