At the moment it seems to be Medium by content length, Reddit by quality mechanism, to some extent Wordpress/Tumblr by ease of entry.

You're right that traditionally CAC is measured in marketing dollars, and that's not what's happening here. But as a real-life experience of the very real cost of conversion: try converting a tech un-savvy user to Steem and see how that goes. You know, that hypothetical neighbor of yours who is a freelance journalist.