I spent about two weeks creating quality articles for the sidebar, personally replying to every submission/comment, manually recruiting anyone who mentioned the LSAT, and reaching out to moderators of related subreddits for links.
Mercifully, a subreddit is a small thing to launch, and after two weeks the place became self-sustaining and grew to 1500 subscribers.
I'm seeing the same thing again with a website I just launched (http://lsathacks.com), which has free LSAT explanations.
Very positive initial comments, but just letting people know about it hasn't resulted in a surge of traffic. Instead, I'm going to have to manually recruit people. Only then will I know if it's worthwhile.
My point is that this doesn't apply just to high growth startups. Almost anything new requires initial unscaleable effort.
Edit: The LSAT is the Law School Admission Test, a logic test required for admission to North American law schools.