Diffusion of innovations
http://en.wikipedia.org/wiki/Diffusion_of_innovations

On the down side of the adoption curve between Late Majority and Laggards and "Market Share " as shown also refers to market saturation; I think the tube market saturation to be around 75% to 85 % with the next Disruptive Innovation coming soon (hold on to your socks) ...

Robbie is basically correct on the Social Media aspect. I see from my own statistics that a Facebook or Twitter referral has twice the value of an organic search engine referral, and more than 200 times the value of a tube bulk bulk traffic referral.

However, there is an overall problem is my statement above -- the volume of available traffic is not factored in. There are maybe 1 Billion 'real' Social Media users and what percentage get off on Internet porn? 10%? That very conservative estimate is a 100 Million person market that may be only partially monetized ...