• by Gavin O'Malley , Staff Writer @mp_gavin, February 17, 2016

Remember earlier this week when I suggested that Yahoo was fumbling Tumblr’s potential?

Unfortunately, eMarketer just released a fresh forecast that appears to support my argument -- and it’s got everything to do with mobile.

“A big factor is that Facebook, Instagram and Twitter are more successful at catering to mobile audiences,” eMarketer analyst Oscar Orozco notes in a new report. The research firm suspects that Tumblr’s usership is already plateauing, and this will mark its last year of double-digit growth.

Of course, it’s not the only social network with slowing user growth, but Tumblr is a fraction of the size of its rivals. Indeed, its reach among Web users is only 8.7% -- far below the 69.9% who use social networks in general, eMarketer notes.

Put another way, Tumblr will have 23.2 million domestic users this year -- less than half as many as Pinterest’s 54.6 million, and less than one-third as many as Instagram’s 89.4 million.

Worse yet, the gap between Tumblr and its competitors is only expected to widen through 2020.

Over the next four years, Tumblr is on track to add a measly 6.2 million users. By contrast, Instagram is set to add 26.9 million users, Facebook will add 14.6 million, and Pinterest will add 12 million.

It’s not for lack of trying. “Tumblr is pushing for new ways of sharing content on its platform, including video, messaging and GIFs -- hoping to increase mobile engagement,” according to Orozco.

Despite these efforts, however, Tumblr’s woes are not helping Yahoo’s broader outlook. This year, eMarketer expects that Yahoo will capture 1.3% of total U.S. digital ad spending -- a slight decrease from last year.