I think the start date, Q4'07 is misleading. I'd like to see the graph since 2005 for example. I believe there was a steep increase of online ad spending in 2006-2008, and this is an adjustment to a "normal" growth rate. Being at the same level as Q4'07 is actually a good surprise to me, I thought it would be well below.
It could just be that advertisers are spending more directly with publishers rather than on AdWords