There is, right now, something of a settling in the online advertising landscape. With Pinterest unveiling its strategy for generating revenue through promoted pins, the owners of the most visited social networks, apps and sites are now public with their strategies for minting billions from digital ads.

With the playing field set, the companies are intensifying their jockeying to woo brands. At stake is a share of dollars going into digital advertising that will soon match, and indeed eclipse, television spending in the next two years, according to eMarketer.

But most of the online publishers face the existential challenge of proving that ads on their sites actually drive sales. And for the companies that have touted their “mobile-first” strategy, and a diminishing reliance on desktop sites, making that case will be difficult so long as the vast majority of purchases aren’t made on mobile.