Pinterest said Thursday it's changing the way it sells CPM campaigns, as marketers will now be able to bid on inventory through an auction process.

Previously, CPMs -- or cost per thousand impressions -- were only available on a fixed priced basis. Meanwhile, the company also added that marketers can now add frequency capping, which will allow them to specify the maximum number of times a person can see their campaign.

Home Depot, JCPenney and General Mills had been testing the new model prior to today's rollout, according to Pinterest.

"The main shift here is that Pinterest is making their inventory have parity with other inventory sources," said Ben Clarke, president at The Shipyard, a full service digital agency. "Marketers are used to being able to buy on a per impression basis and be flexible with CPM as well as set things like frequency caps. These changes make Pinterest easier to include in current media plans as an extension of the existing strategy rather than having to create a 'Pinterest only' buying model."

Thursday's move also underscores the overall trend to make social media advertising more performance-based, said Marco Bertozzi, global chief revenue officer at Publicis Groupe-owned Performics.

"Historically, social has been a branding channel for advertisers, promoting awareness and boosting share of view," Mr. Bertozzi said. "But over the past few years, advertisers -- and social networks -- have changed this thinking. Social is no longer just a branding channel, but also performance channel that can be very effective in driving clicks, leads and sales, engaging consumers at specific stages in the purchase path."

At the same time, the push to add frequency capping may lure more performance marketers.

"Pinterest is an intent engine that people leverage within different stages of the discovery process. Depending on where consumers are in this purchase path, advertisers may want to increase or decrease ad frequency," Mr. Bertozzi added. "This is another lever to pull to drive advertising efficiency."