Two years ago when we kicked off development of our content recommendation widget mobile was high on our mind and "mobile first" was the approach we preferred. Fast forward two years and we spend very little time thinking about mobile today. The main reason for such thorough change of hearts is the customer pivot we had done over the past two years. If two years ago we were mostly focused on solving problems of publishers, we are now 100% focused on problems of advertisers. And it turns out mobile is mostly problem of publishers, not of advertisers.
Over the past couple of years publishers saw 50% of their traffic become mobile, while large majority (90% and more) of their revenue still stems from users using a stationary computer. Fortunately for publishers digital advertising is still growing fast enough that growth in price per desktop impression offset the declining desktop usage, but the fast growth in online advertising that publishers hoped would replace dwindling offline revenue hasn't materialized. Publishers were too slow in finding ways how to engage users on mobile with something more compelling than a miniature banner ad and, even more importantly, they completely forgot about the audience targeting on mobile which made mobile banners pretty much useless for advertisers.
I've discussed her information consumption habits with my 14 years old cousin yesterday and it's obvious her generation can be reached only on mobile. So eventually also advertisers will have to spend more of their advertising budgets on mobile, too. But until traditional publishers get their act together all benefits of mobile advertising will be reaped by Facebook, Snapchat, and other such services alone.