You get what you pay for. Free traffic is the worst, most uncontrollable traffic on the planet and ten or twenty times the work. Almost half the traffic on the web now is bot traffic. The rest of free traffic is mostly disinterested users that are not willing to fulfill conversion requirements and only click on banners as a matter of browsing practices.
Paid traffic, good trackers, and learning to interpret the data is the fastest and most reliable means to develop a positive ROI. Especially if you are doing content marketing, but holds true for the rest of the industry. The businesses you sell banner and contextual ad space to are going to rely on you providing quality targeted traffic.
Yeah, to be honest, I'm near my wit's end with free traffic at this time and it's probably one of the first things I'd spend money on next to acquire targeted visitors. Right now, I'm mainly using Jay's methods of building a Tumblr or Pinterest following and directing them to my blog as my main free traffic tactic (although I may also consider guest posting on other similar but more trafficked blogs for content marketing.)