US mobile ad spending on social networks will reach $4.5 billion by 2018, according to a new ForecastView from Forrester Research.

“[Forrester] expect[s] the spending on mobile ads to grow nearly twice as fast as the spending on desktop ads on social networks. We believe that the future of social is mobile,” writes Jitender Miglani in a new blog post announcing availability of the forecast to clients.

In the post, Jitender outlines why mobile will be a key driver to social media spending growth: More users are accessing these sites through mobile devices, share of time spent on social networking sites on mobile devices is increasing, mobile ads provide better ROI for marketers, mobile ads earn more money for social media vendors than other platforms, and the mobile ad ecosystem will continue to evolve.