Facebook's shares soared Wednesday after co-founder Mark Zuckerberg told analysts the shift to mobile would be "fundamentally good" for the company.

Facebook's shares rose over 20% in early trading Wednesday to more than $23, the first time most investors had an opportunity to buy shares since Tuesday, when Facebook released its second set of results as a public company. The social network company has struggled since its disastrous initial public offering in May. Shares that launched at $38 crashed on the second day of trading and have hovered around $20 for weeks.

On a conference call with analysts, Zuckerberg said Facebook's six-month old mobile ad business now accounted for 14% of advertising revenue, about $150m. The number of mobile monthly active users rose 61% to 604 million, the company said Wednesday.

Mobile is the fastest growing way that people access the service and both the company and analysts have expressed concern that Facebook will struggle to make ads work on mobile devices.

Zuckerberg told analysts: "I think our opportunity on mobile is the most misunderstood aspect of Facebook today," he said.

Mobile sales helped Facebook's revenue rise 32% to $1.26bn in the quarter that ended September 30, slightly topping Wall Street's expectations. But despite the rising revenue, Facebook still made a loss of $59m as costs soared 64%.

Citi Research upgraded the stock to buy Wednesday. "What investors have for the first time since the Facebook IPO is fundamentals acceleration with a reasonable valuation," analyst Mark Mahaney wrote in a note to clients.

Facebook's IPO was marred by a series of errors, but news of its problems in mobile were probably the largest single factor in the dramatic share price fall. In regulatory filings the firm warned that mobile ad sales were negligible.

The firm is also being sued by investors who claim Facebook unfairly warned some big investors about its mobile issues while withholding those fears from others.