In addition, you can see that over this time period the average number of monthly users has grown to about 197 million in the US, although the rate of growth of users and minutes has been roughly the same, meaning users are not really using the web much more intensely than before. The daily UV figures are also fairly static.

Facebook and Youtube are large and have seen mega-growth over the last three years, and yet they are probably underperforming in terms of their revenue per user hour figures. Any change in their ability to monetize their traffic will certainly have a huge impact in the overall online advertising figures. Figures below are in millions of minutes per month.