• by Erik Sass , Staff Writer @eriksass1, October 15, 2013

Social media advertising is already a good-sized business in revenue terms, but it is about to get even bigger, according to a new forecast from ZenithOptimedia, with a considerable amount of this growth coming from the rise of social mobile advertising.

ZenithOptimedia (ZO to their friends) estimates total social media advertising spending will reach $4.6 billion this year, up 35% from $3.4 billion in 2012. Within these figures, spending on social media advertising targeted to mobile devices will soar 170% from around $275 million to $740 million.

But that’s just the beginning: looking ahead, ZO sees social media ad spending climbing to $8.22 billion by 2015, which suggests a cumulative annual growth rate of around 35% over the next two years. Meanwhile mobile advertising will jump 86% to $1.37 billion in 2014, then 62% to $2.22 billion in 2015, at which point it will contribute 27% of total social media ad spending.

While impressive in percentage terms, in dollar terms social media’s projected growth rate actually tracks pretty closely with ZO’s forecasts for increases in U.S. spending on paid search, display, and online video: ZO sees paid search increasing from $14.9 billion in 2013 to $18.9 billion in 2015, while display will increase from $7.6 billion to $11.3 billion, and online video will increase from $5.2 billion to $8.3 billion. In other words, all four subsets of digital media will add $3 billion-$4 billion in revenues over the next two years.

Of course, there are still some hurdles to be overcome: above all, social media platforms will have to demonstrate an ability to measure ROI convincingly -- something that is still very much a work in progress.

On that note, in August Forrester released the results of a survey in which 38% of marketers surveyed identified ROI as one of the top three challenges they face, followed by measurement issues, identified by 35% of respondents. And last year I wrote about a survey of 329 brand marketing executives by PulsePoint and the Economist Intelligence Unit, in which half of the survey respondents said lack of standardized metrics for ROI was a big impediment for social media campaigns.