Paul Sloan/CNET

PALO ALTO, Calif. -- Facebook CEO and co-founder Mark Zuckerberg addressed an adoring crowd at Y Combinator's startup school today, speaking confidently about Facebook and describing a world in which people will share a whole lot more than they do now -- via Facebook and other social companies.

"It's sort of a social-networking version of Moore's Law," said Zuckerberg, who was interviewed by Y Combinator co-founder Paul Graham. "We expect this rate [of sharing] will double every year. ...So in 10 years from now, people will be sharing about 1,000 times as many things as they do today."

That, of course, is what Zuckerberg and the newly public Facebook are banking on: That the company's 1 billion plus userswill keep using Facebook not just for keeping in touch with friends but also for interacting with brands.

Today's interview wasn't about Facebook the business however. This was a crowd of huge fans -- some 1,700 entrepreneurs and aspiring entrepreneurs who came from across the country, largely to get inspired by Zuckerberg, who was the featured speaker. They packed into Stanford Memorial Hall auditorium, some of them lining up hours in advance to snag a good seat.

So Graham, who runs the most influential startup incubator anywhere, didn't ask about the stock price or about the impact that Facebook's messy IPO has had on startup funding in general. He didn't ask about the ad business. He didn't even once mention the word "mobile," which is Facebook's key challenge now that 600 million Facebook users are accessing the service on their smartphones.

Instead, they talked about Zuckerberg's days at Harvard, where he majored in psychology and began building Facebook in his dorm room before dropping out in 2004 and moving to Palo Alto to go at it full time. Graham asked how much Facebook's first server cost -- answer: $85 -- and Zuckerberg said in the beginning he never spent money Facebook didn't have. He stressed that he didn't start Facebook because he wanted to build a company -- something he talks about in his letter to shareholders -- but because he "really wanted this."

"I felt this need acutely," he said of wanting to build a service to connect people. "I started building Facebook because I wanted it at college, which is one of the ironies, since I then left college."

He talked about this to stress that he thinks entrepreneurs need to focus on big, meaningful problems that they're passionate about. And while he didn't take a shot at Silicon Valley, as he did last year, he did complain that startups are generally tackling small issues.

"A lot of companies I see are working on small problems," he said. "Companies getting started now are trying to copy stuff others are doing and just aren't going to be successful."

He told the audience to work on something "that's fundamentally" meaningful and said that he's always had a "fear of getting locked into doing things that aren't most impactful."

"Explore what you want to do before committing," Zuckerberg said, "and keep yourself flexible."

As for Facebook, he argued that the social network is fundamentally changing human behavior by expanding the number of people we can keep in our social circles -- offline and on. When people sign up for Facebook, he said, they initially connect with about 150 friends, because that's generally the number of people that one can keep up with.

But over time, he said, people on Facebook add more and more friends.

"One definition of technology is that it extends human capability," Zuckerberg said. "A social network extends people's real social capacity."