Let we discuss only onr point which is the price of the click.
Facebook and google, you can set the price $0.01 per click.
Adhitz and advert, the smallest price is $0.04 per click.
Now, We now that the facebook is the largest website in the internet but why thr price of advertising via it cheaper than adhitz.
My answer is : Facebook is a social site, but adhitz is commerical site specialises in PTC ads.
Conclusion: if the ad network is a commerical sites in the first hand. Of course, the price must be bigger.
An essay from this blog: http://www.exchanges-traffic.com/
Please, If the link is not permitted, remove it