If I got 1,000 visits per month to an ebook site through content marketing, I could only count on about 2% joining my email list, and around 9% of those ever buying (3% across three pitch emails). Multiplying those, content marketing would only drive 1.8 sales per month. Clearly some people are finding much more success than that - which of these numbers is off?
Unfortunately all the numbers are off. Content builds on itself in an accumulative fashion. Much of it is unquantifiable, but the success comes as breakthrough success after building your authority over time (atleast 18 months of consistent publishing content.)
Those 1000 monthly visitors should be growing consistently every month over time. If it is not growing you are not producing valuable enough evergreen content.
The list should be growing your traffic, not just sales...it should be a referral machine.
Are you engaging your audience when they interact with your content across various social channels? Is your content being linked to and shared organically? If not, you aren't really doing effective content marketing.
Are you constantly testing your emails and landing pages for improvements to conversion rates, analyzing which content is producing the most signups, sales, etc... If that is not growing exponentially you are doing something wrong.
Are you advertising your best content using native ad platforms like stumbleupon, outbrain, taboola, yahoo gemini? Why not? If your content is not valuable enough to drive signups and sales when you promote it, you are doing content marketing wrong.
Are you adapting your content into multiple media types and syndicating it? (i.e. a blog post can be shortened into bullets for a slideshare presentation, which can be used a slideshow video on youtube, and an audio reading of your post can be on soundcloud, syndicated as a podcast, etc...
Are you doing blogger outreach to generate interest in your content? Are you guest blogging to leverage other peoples audiences?
Content marketing is an accumulative effort, which when done right builds on itself accumulatively and snowballs into breakthrough success (i.e. look at moz.com and copyblogger.com both massive startups which started with simple content marketing.
I hope this helps.