I think the issue is not so much that they'll continue to grow -- I don't anticipate a majority of the United States, for example, becoming ideologically identical to the "SJW"-type people on Tumblr.

The issue is that a very small but loud group of online activists are becoming able to have disproportionate influence. It's easy for online mobs to form on sites like Twitter or Tumblr, which have messaging systems that promote reblogging/retweeting/"signal boosting" and are neither good for nuanced discussion nor corrections. Companies and organizations depend so heavily on Internet social media for their PR that they quickly bow to mob pressure in terms of self-censorship or firing "problematic" employees.

So it's not so much that the "social justice activists" will spread in terms of population, it's that they can have a chilling effect where the other side doesn't speak out, out of fear of the nuclear option of online shaming being used against them. This can lead to one side dominating the conversation despite being unpersuasive.