NEW YORK – Facebook grew its advertising revenue by 64 percent in the third quarter, helped by a boost in mobile ads that are becoming an increasingly large chunk of the social networking giant’s overall advertising business.

The steady increase indicates that Facebook has succeeded in steering advertisers to its mobile platform at a time when most of its users are using Facebook on phones and tablets. Investors were initially worried about the desktop Web era-born company’s ability to succeed in mobile advertising, but those concerns are long gone.

Though Facebook’s results surpassed expectations, investors sent the company’s stock down sharply not long after the results came out, possibly spooked by comments during a conference call that 2015 will be a “significant” year for expenses. The company said it expects costs to grow by 55 percent to 75 percent next year as it ramps up investment in its work force, growing existing products and new areas such as WhatsApp, Oculus and video.

This year, Facebook spent $22 billion in cash and stock to buy the messaging service WhatsApp and about $2 billion to buy the virtual reality company Oculus.