Facebook is pulling in more money from soaring mobile usage of the huge social network, but not enough to get Wall Street excited.

Facebook on Wednesday reported a profit of $64 million in the fourth quarter, a steep drop compared with $302 million in the same period 2011.

Revenue grew 40 percent to $1.585 billion but expenses jumped by 82 percent to $1.06 billion.

Facebook also said the number of mobile users jumped 57 percent from a year ago to 680 million, and for the first time surpassed the number using personal computers, at 618 million on average for December 2012.

About 23 percent of Facebook's ad revenue came from mobile devices, affirming the company's efforts to find ways to make money from the unrelenting trend of relying on smartphones or tablets to get to the Internet. That figure was up sharply from 14 percent in the third quarter.

"In 2012, we connected over a billion people and became a mobile company," chief executive Mark Zuckerberg said as the California firm released its quarterly results.

"We enter 2013 with good momentum and will continue to invest to achieve our mission and become a stronger, more valuable company."

Facebook played up the number of monthly active users, especially those connecting to the social network on smartphone or tablet computers.

Approximately 1.06 billion people used Facebook monthly in a 25 percent increase from the prior year.

Wall Street's early response in after-hours trade was to push the stock down, but only modestly. The shares, which fell by half following their debut in May and have been on a steady climb, dipped 1.1 percent in electronic trades to $30.88.

Jon Ogg at 24/7 Wall Street said Zuckerberg is trying to sound more like a public company CEO than a social media geek.

But he said investors "May raise some brows after seeing the margin compression, although some of this was known due to hiring and expense items."

Zuckerberg in said in September that the social network giant is focused on mobile devices and should be seen as a smart bet despite a disappointing stock market debut.

"It is really clear from the stats and my own personal intuition that a lot of energy in the ecosystem is going to mobile, not desktop (computers)," Zuckerberg said during an on-stage interview at a TechCrunch Disrupt conference in San Francisco.

"That is the future," he continued. "We are going to be doing killer stuff there."

Zuckerberg was adamant that the company was being underestimated and was on track to make "more money on mobile than we make on desktop."

His appearance at the conference marked his first public interview since the massive public offering on May 18 that was hotly anticipated -- but ended up being a flop.

Facebook shares quickly sank to less than their value since the IPO at $38 a share but had been climbing their way back prior to the earnings results on Wednesday.

Zuckerberg, who holds a controlling interest in Facebook, said at the conference that the company has repeatedly seen public opinion swing from unreasonably bright to overly dark.

"I would rather be in the cycle where people underestimate us," Zuckerberg said. "It gives us good latitude to go out and make big bets. I think a bunch of people are underestimating us."