This report is worth looking . I like this graph -- it supports my thinking on verticals.
This quote is directly from the report:
"The breakout of spend within the vertical category saw a significant shift in 2006, with News, Reference, Business, and Entertainment all up considerably over 2005. While communities and social networks have sometimes been seen as a service looking for a revenue model, spending in that category was up 69% year-over-year and 216% since 2004. As a result of that increase, the Community category tied with Entertainment as the vertical with the largest share of billings.