Naturally, a complete answer will depend on your advertising goals, and any serious advertising campaign will want to be prefaced with experiments and cost/benefit analysis, etc.
That being said, you can check out these ad networks. I get the sense that they're somewhat stilted in their demographics, but whatever:
There are several different metrics we use as constraints.
1. Has to be a cost per click ad model. It can not be impression based.
2. Two goals: Cost per visit under $5. Cost per engagement $10.
3. We need the ability to add several layers of demographic targeting.
4. The idea behind exploring new channels is to engage users with great content. Google Search does a good job, but the content may be to to niche.
That being said, you can check out these ad networks. I get the sense that they're somewhat stilted in their demographics, but whatever:
- Reddit
- StumbleUpon
- Digg
- Pinterest
- I think Demand offers display advertising?
- Project Wonderful