SAN FRANCISCO, CA: Advertisers are increasing their investment in mobile to the extent that this accounted for 49% of US spending on paid search ads during the fourth quarter of last year.

According to a report from Marin Software, a cross-channel performance advertising platform, this was up from 42% during the third quarter, as consumers continued to make increased use of smartphones and tablets in their purchase decisions.

Smartphones and tablets also accounted for 39% of clicks on paid search ads, while the click-through rate (CTR) for search ads on smartphones was 38% higher than the equivalent for desktop ads.

Further, Marin found that 31% of conversions on search ads came from mobile devices.

In examining cost-per-click (CPC), Marin reported that search ads on desktops retained the highest CPC, whereas mobile ads on Facebook ranked lowest.

It suggested that the lower CPCs on display and social ads represented an opportunity for advertisers to leverage proprietary search intent data to target and reach key audiences on these channels with more cost effective campaigns than search.

The Facebook statistics made interesting reading. The social networking site now regards itself as primarily a mobile business and Marin found that 63% of Facebook ad clicks came from mobile devices. 

But mobile accounted for only 34% of conversions, defined as a sale, download or completed lead generation form.

Advertising Week delved deeper into the figures and noted that during Q4, the conversion rate of Facebook's desktop ads was almost four times that of its mobile ads, at 1.1% against 0.3%.

And it compared this with Google and other search engines, which returned a 10.1% conversion rate for desktop ads and 6.6% for smartphones and tablets.

"The disparity points to the value in Facebook advertising being closer to television or print than performance-marketing channels," according to Matt Ackley, Marin Software CMO.

"Clicks on mobile ads are greater than desktop ads, indicating Facebook mobile ads are a great way for a retailer to build brand awareness and reach consumers on mobile devices," he said.

"Consumers are not device or channel exclusive. They move from search to social sites."

Data sourced from Market Wired, Advertising Week; additional content by Warc staff