incrediBILL - 5:47 pm on Jan 27, 2010 (gmt 0)

There must have been a lot of advertisers shut down in order to make CTR fall by 50%

I'm not sure how advertisers shut down and CTR decline go exactly together.

CTR declines would tend to fall when people aren't buying, such as running a site about cars and car sales plunging would likely follow suit CTR would tank.

The other thing to tank CTR is site targeting and Google has gone off the deep end in that dept. as far as I can tell which is definitely a CTR killer.