NEW YORK (AP) — Worldwide spending on mobile advertising is expected to reach $31.5 billion this year, a 75 percent increase from 2013 thanks largely to Facebook and Google, according to a new report by research firm eMarketer.

By the end of this year, eMarketer expects mobile to account for nearly a quarter of total digital ad spending, which is estimated to reach $137.5 billion. That’s up from 15 percent of the $119.8 billion digital ad spending total in 2013.