Roughly one-fifth of the dollars marketers spent on Facebook ads in November and December went to reaching consumers accessing the social network on a mobile device, according to Kenshoo Inc.

The company, which provides software to help retailers manage social media, paid search and online marketing programs, based its findings on data from Kenshoo’s clients, which include 10 retailers in the Internet Retailer Top 500 Guide. Among them are Wal-Mart Stores Inc., No. 4, and Sears Holdings Corp., No. 8. The report also includes data from Kenshoo’s clients in other industries, such as entertainment and finance.

With 20.3% of all ad spending on Facebook devoted to mobile devices in November and December—a six percentage point jump from the previous two months—it is clear to Kenshoo that the social media advertising landscape has changed dramatically from a year ago, when Facebook had yet to offer a single mobile advertising unit. “The social advertising landscape is radically evolving,” says Todd Herrold, senior director, product marketing for Kenshoo Social. “These are really significant changes and they’re happening very quickly.”

Facebook last year introduced multiple mobile ad units, all of which appear in consumers’ news feeds. Reaching consumers in their news feeds is essential, says Facebook, because that is where consumers spend 40% of their time on the social network.

Because all the mobile ads appear on the news feed—and the social network has said it doesn’t want to overwhelm consumers’ news feeds with advertising—there is a limited supply of mobile ad units, says Herrold. That’s different from the desktop, where Facebook offers display ads on the right side of the page, as well as full-page ads that appear when a shopper logs out from the social network.

The limited supply of mobile ads has driven up the cost-per-click of Facebook ads, he says. The cost-per-click for a desktop ad on Facebook is 81 cents, compared with $1.38 for mobile ads.