I have visited a lot of websites and noticed many of their Scratchback widgets are empty, that is no one paying for it.
This affiliate marketing website that I found has about 22,000 unique visitors per month, based on Compete Analytics, and guess how many tippers are there? 1, Yes, ONE only.
Who are the tipper? Creator of Scratchback.

Why is it so? Some of my guesses:
o The blog readers don’t notice the widget
o Most people don’t tip for plain links.
(SEO Link Building is a $1 billion industry worldwide. Google, Yahoo, and MSN Revenue from CPC alone exceeds $21 billion worldwide annually.)
Links in Javascript with rel=no follow just does not cut it
o Introducing our Server-side scratchback that will give you more recurring income, why because our widget links have no CONDOMS on it.
