The cost is so low that I am sure advertisers are drawn to the concept just to see if it works. How low? Try $1 a pixel for five years. In the case of PixelBay the company touts the fact that it will drive traffic to its website in large numbers. A look at the Alexa Rankings of the site show that while traffic spiked for a while, it is on the way back down now.
Here is anarticle on the topic worth reading and this is where I found this quote that I agree with: "Things that have no benefit to the consumer don’t last very long. They can survive on curiosity and novelty for only so long," said Jim Nail, analyst at Forrester Research,