If you're not considering dedicating a larger portion of your advertising budget to mobile, you're behind the times.

A new report by research firm eMarketer predicts worldwide spending on mobile advertising will reach $31.45 billion this year, a 75 percent increase from 2013. Mobile advertising will account for nearly a quarter of total digital ad spending, according to the report.

What companies are leading the trend? Not surprisingly, eMarketer forecasts Google will be on top in 2014 with 46.8 percent of net mobile ad revenue worldwide. Facebook is predicted to be next with 21.7 percent, while Twitter ranks a distant third with 2.6 percent. Those companies occupied the top three spots, in the same order, in 2013 as well.

Mobile ad spending around the world increased by 105 percent in 2013, according to eMarketer. The rapid pace of growth is not expected to last, however. The report predicts that by 2018, mobile ad revenue will climb to $95 billion but the growth rate will be only 22 percent.

Growth in ad spending will decline as the the digital ad space matures, according to TechCrunch, as well as from companies adopting other revenue models, such as game makers relying on in-app purchases rather than ads.