For free traffic, my best suggestion is Facebook groups.
I've been having a lot of great success promoting and sharing stuff on niche-specific Facebook groups.
Even classified ad groups on Facebook groups can lead to traffic if you know how to share the right pieces of information.
It’s all about getting people to your page and then, from your page, getting them to your target traffic.
This is something that you have to scale up slowly so you can figure out which groups are worth the effort, but the time you invest figuring things out is definitely worth it.
You can't just go in there, blast a thousand groups, and expect positive results—it doesn’t work that way.
You have to start low and slow and just really pay careful attention to the results that you are getting.
Build on your success; ditch the failures.