I actually always liked this article which deflated those numbers and also called out that the $20 billion was not part of any study.
http://www.forbes.com/2001/05/25/0524porn.html
I love this quote, because it's so true.
"Skepticism is in order, though, because as David Klatell, associate dean of the Columbia Graduate School of Journalism notes, "[Pornography] is an industry where they exaggerate the size of everything."
Back in the day when other programs used to share their numbers with us verbally we always had a rule to divide by 2 if not 3 depending on who you were talking to.
I think the $ around the porn industry as a whole is irrelevant and the more important factor is that pretty much 99% of men enjoy watching adult entertainment is the more telling factor. It's a huge market.
~Alli