Zuckerberg said mobile was a "massive opportunity" for Facebook. "The big thing is obviously going to be mobile," Zuckerberg said. "There are 5 billion people in the world who have phones, and a billion people using Facebook. There are actually already 600 million people using Facebook on phones, so that's growing really quickly."

Discussing the company's and stock price, Zuckerberg again brought up mobile, saying figuring out how to monetize mobile is going to be one of the bets he'll be making.

"A lot of it over the next few years is going to come down to mobile," he told Businessweek. "We already know that people who use Facebook on mobile use it more, spend more time on it."

What Zuckerberg didn't talk about was how Facebook is going to monetize mobile. And industry analysts say part of the reason for that may be that he's actually still trying to figure it out.

"The problem is no one other than Google is getting mobile advertising to work and with Google only through traditional search," said Rob Enderle, an analyst with the Enderle Group. "It is an issue with the device. It doesn't lend itself to traditional forms of advertising... The screen is simply too small and the potential for user aggravation too high."