The research firm suggested that Tumblr's reach among internet users is just 8.7%, rising to 12.5% among social network users.
In comparison, fully 87.8% of social network users have a Facebook account; comparable figures for Instagram and Twitter are 48% and 30.6% respectively, while Pinterest comes in on 29.4%.
In terms of Tumblr's actual user numbers in the US, these stand at just over 23m and are predicted to rise to 29.4m by 2020, but at that rate of growth, it will simply be falling farther behind its competitors, eMarketer said.
While Tumblr is forecast to add 6.2m US users over the next four years, Instagram will add 26.9m, with Facebook up by 14.6m, Twitter up by 13.9m and Pinterest up by 12m.
"A big factor is that Facebook, Instagram and Twitter are more successful at catering to mobile audiences," said Oscar Orozco, eMarketer's forecasting analyst, although he added that Tumblr was attempting to address this.
"Tumblr is pushing for new ways of sharing content on its platform, including video, messaging and GIFs – hoping to increase mobile engagement," he said.
Yahoo is also reported to be reversing a decision it made only last year to merge its own ad sales team with that of Tumblr.
"We were distributing Tumblr sponsored posts across Yahoo content in their native stream, but very few advertisers actually wanted it because it's a different audience," a source told Business Insider.
"And the content on Tumblr was very different from the content on Yahoo."
Data sourced from eMarketer, Business Insider; additional content by Warc staff