I will be participating in an ad:tech panel this afternoon with the title “Digital Economy 2010: A Robust Recovery or Another Year of Trench Warfare?"  I have to admit that I am not entirely sure what that means, but in a prep call with moderator Mark Mahaney we came up with some fun questions.  The one that is most interesting is whether we may be facing a long-term decline in the percentage of GDP that is spent on advertising.  I am pretty sure that I saw a statistic to the effect that advertising as a component of GDP is the lowest it has been in a long time, but right now don’t seem to be able to find it (so there is a small possibility that I dreamed it up).   There are two completely opposite ways such a decline might happen.  It could be a sign of advertising getting less effective and hence being used less.  Or it could arise if advertising is getting much more efficient and a smaller amount of it can sustain the same demand.  In either case though it is possible to see a decoupling of economic growth from advertising growth.