Costolo’s comments, as reported by Reuters, came Tuesday during a tech conference hosted by The Economist Group. Bloomberg recently reported that sources told them Twitter expects to earn $1 billion in sales by 2014, a number closely tied to the comany’s mobile advertising success.

Twitter’s advertising is integrated into user newsfeeds, in the form of promoted Tweets and hashtags paid for by advertisers, making it well-suited for mobile platforms.

In contrast, Facebook ads are much more desktop-focused, and the company has struggled to find an advertising strategy that works for mobile. In 2011, the company earned 85 percent of its $3.7 billion in revenues from advertising, but none from mobile, even though 425 million of 845 million monthly active users in December accessed the site  through mobile technology.

In contrast, Twitter allows advertisers to purchase ads specifically for display on iPhone or Android devices. As of January, about 55 percent of Twitter users accessed the service through mobile devices.

However, it’s not just Facebook that has a mobile advertising problem, said Mary Meeker of Kleiner Perkins in a recent presentation at D: All Things Digital. While mobile traffic is growing rapidly, most companies still think desktop ads are worth about five times the mobile equivalent. And companies who are making money off mobile ads are still making less than they do off desktop equivalents.

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