The social networking service made the mistake of focusing on the next generation of Web technology, called HTML5, which is designed to let applications work across many types of hardware, but it didn't let Facebook Inc.'s applications have the sophistication of those designed specifically for devices such as Apple Inc.'s iPhone, he told an annual conference in San Francisco sponsored by the TechCrunch technology blog.
"We had to start over and rewrite everything to be native [to devices such as iPhone]," Zuckerberg, 28, said in a half-hour onstage "fireside chat" before a standing-room-only audience. "We burned two years. It may turn out it was one of the biggest if not the biggest strategic mistake we made."
But the 8 1/2-year-old company has shifted its focus and hopes to have a new Android smartphone and tablet computer application "soon."
"It'll be ready when it's ready," he said.
He said Facebook had no intention of developing a smartphone of its own.
"It's so clearly the wrong strategy for us," Zuckerberg laughed.
It "sounds juicy" but wouldn't add enough customers to Facebook's 955 million active users to "move the needle," he said.
The company is figuring out how to best integrate advertising into the mobile experience, he said.
Zuckerberg, appearing in a gray T-shirt without his trademark hoodie, told the TechCrunch Disrupt conference he was unhappy about the nearly 50 percent drop in Facebook's stock price since its May 17 initial public offering at $38 a share.
"The performance of the stock has obviously been disappointing," he said. But the company should be measured by its lasting effects on the world, he said.
"Ten, 20 years from now, the legacy of this company should be, we have connected everyone in the world," he said. "That's a lot."
TechCrunch founder Michael Arrington asked Zuckerberg if the stock drop has caused morale problems.
"Well, it doesn't help," he said. But he said Facebook employees are accustomed to ups and downs.
"Facebook has not been an uncontroversial company in the past," he said.
Facebook stock, which hit a low of $17.73 a share on the Nasdaq stock market Sept. 4, closed up 3.3 percent at $19.43 a share Tuesday and rose to slightly more than $20 a share in after-hours trading.
Zuckerberg said Facebook also intended to develop search-engine technologies, an area dominated by rival Google Inc.
"We do on the order of a billion queries a day -- and we're not even trying," he said, adding later he "was being facetious" with his "not even trying" comment.
"We have a team working on search," he said, explaining the team was working to make search engines provide more sophisticated results, including names of people working at a company where a person may be looking for a job and "what sushi restaurants have my friends been to in the past six months and liked?"
Arrington also asked Zuckerberg if he is still having fun.
"Yeah! For me, though, it's not really about fun, it's about mission," he said in remarks cited by Forbes. "I personally would rather be in this cycle where people underestimate us. I would rather be underestimated. It gives us a chance to go out and do great things.
"Some days are hard, some days kick ass," he said.