What do shag carpets, landlines, and display advertising all have in common? The heyday for each has long since passed. And in the case of display ads, a shiny and new alternative — one that blends word-of-mouth buzz with high frequency — has emerged: influencer marketing.

It’s easy to understand why influencers have more reach than website banners or YouTube adverts. Our company did a joint study with Nielsen on consumer conduct, and we found that 82 percent of...