Growing up, Ben Silbermann liked to collect things: stamps, insects, you name it. At Yale, he studied medicine and political science. After college, he got a job as a consultant in Washington, D.C.

Not exactly prototypical start-up material, but today he's CEO of one of the world's top social networks, Pinterest, which he co-founded and is valued at a staggering $1.5 billion.

Silbermann's first foray into the Silicon Valley tech scene was cartoonishly wide-eyed. He'd feel bolts of excitement reading about start-up company hires and funding round announcements on tech blogs.

"I felt like it was a story of my time that I wasn't a part of," Silbermann told a rapt audience of hundreds at the South by Southwest Interactive Festival in Austin, Texas, this March, where he did a rare on-stage interview. (Heck, any interview with Silberman is rare. He declined to be interviewed for this article, saying he and his small team were working too hard to be interrupted.) Silbermann decided he should get his start at Google, and landed an entry-level job at the company’s display-advertising group.

The route to running Pinterest, which has nearly 20 million unique visitors a month and $138 million in funding, began in November 2009, when he and a college friend, Paul Sciarra, along with Evan Sharp, who had been studying architecture, started working on a site on which people could show collections of things they were interested in, on an interactive pin-board format.

But at first, going was slow. So was user-adoption. "It was, like, catastrophically small numbers," Silbermann said. "After nine months, Pinterest was still under 10,000 users, and a lot of them weren't using it every day."

Silbermann and his team labored long and hard over the most miniscule site details, such as typeface contrast. They fully coded dozens of versions of the pin screen before selecting and launching one in March of 2010.

Today, that final design (a grid of bulletin boards), and its ease of use, has made Pinterest wildly popular around the world--particularly in the United States and Asia. It hosts boards from a wide range of people: men's style enthusiasts, architects, yogis, and suburban moms pinning DIY projects and ruffled bloomers on chubby toddler butts. "There are a lot of people who use it for what I call 'core lifestyle activities,'" Silbermann said with a smile. At South by Southwest he seemed proud that unexpected satire boards have emerged, such as ones mimicking the Romney family's style, and that museums, such as the San Francisco MoMA, have joined.

The Pinterest team (which Sciarra, who had been listed on SEC documents as the company's CEO, left in March) has been less forthcoming about plans to build the site into a profitable company, but gave one hint recently: "It has to be something that speaks to the arc of the product itself, which is discovering things [users] didn't even know they needed," Silbermann said.

Just last month, a study found Pinterest surpassed Facebook in terms of retail outlets or brands that fans "follow" or "like." A survey by PriceGrabber, a price comparison site, found that 21% of Pinterest users polled had purchased items they spotted on the site.

There's a lot that remains to be seen in Pinterest's future. Two years into heads-down hard work on building out the site, Silbermann has some life-balance advice for future CEOs: "It's important to have people in your life who can talk to you about other stuff: life, sports. It's going to take a long time to build a big product that'll change the world."