How big is affiliate marketing on the internet?

I am trying to guage the size of the affiliate marketing on the internet, in $ amount. I am helping a friend write a business school paper.
* what % of rev comes through affiliates, typically?
* How many affiliates are out there? How much does make on average?
* How has this market grown over the years? down/up?

any other statistics with sources would be greeeeattt!
I am trying to guage the size of the affiliate marketing on the internet, in $ amount. I am helping a friend write a business school paper.
* what % of rev comes through affiliates, typically?
* How many affiliates are out there? How much does make on average?
* How has this market grown over the years? down/up?

any other statistics with sources would be greeeeattt!

– no generic answers please….statistics are required.