Can Pinterest convince brands to buy mobile?

And mobile for Pinterest means smartphones specifically, according to comScore, which added that smartphones have been more widely adopted by consumers than tablets.

Pinterest’s mobile audience would seem to be an advantage considering how often brands and agencies talk about being “mobile-first.” But in actuality, mobile advertising spending lags woefully behind mobile consumption. This discrepancy was on full display when Mary Meeker, partner at venture capital firm Kleiner Perkins Caufield Byers, included it in her “Internet Trends 2014” presentation at the Code conference in California last week.

Meeker pointed to IAB research that says mobile accounts for 20 percent of media consumption but just 4 percent of ad spending. And even though research eMarketer released in April says the mobile gap is slightly smaller — 23.8 percent of time spent and 10.7 percent of ad spending — it’s apparent that brands have yet to embrace mobile like consumers have.

For Pinterest, this means convincing brands about the value of mobile right as the company is beginning its overall ads pitch.

“Pinterest needs to answer: ‘What does this mean, and why should my brand be there’?” Erin Kienast, director of mobility at Starcom MediaVest Group, said. “I don’t think they’ve really done that, yet because it’s a platform that people have been really interested in for awhile.”

Pinterest has certainly been playing up its mobile presence. A pitch deck Pinterest circulated to media buyers earlier this year showed Pinterest touting its mobile reach, especially among women. But how that mobile reach translates into a superior ad experience for brands remains unclear.

A common refrain among ad tech execs trying to close the mobile spending gap is that Pinterest has yet to prove the return on investment of its mobile ads.

“Brands don’t want to be the first [on mobile], but as more and more platforms come out, this is where their audience is,” Kienast added. “They’re still going to be wary until they can prove ROI, which is kind of the elephant in the room of mobile conversations.”

Jill Sherman, group director of social and content strategy at DigitasLBi, said Pinterest’s substantial mobile audience could actually be a major selling point to brands. Pinterest’s smartphone app shows two columns of images, making it less visually overwhelming than the desktop version. It also means mobile ads would be harder to ignore.

One of Pinterest’s mobile selling points will likely be its ability to drive sales on brands’ mobile sites, Sherman said. EMarketer projects the number of mobile shoppers in the U.S. to increase from 124.8 million this year to 181.5 million — or 86 percent of all smartphone users — by 2015.

Pinterest’s ability to drive those sales is predicated, however, on retailers creating more sophisticated mobile commerce experiences. As such, Pinterest’s mobile challenges reflect that of the Web at large: Consumers are flocking to mobile in shockingly large numbers, but brands are still slow to follow them there.