This change was good for advertisers as it avoids accidental clicks. However, it had major effects on publishers that have optimized their sites for such occurrences. Markus Frind of Plenty of Fish experienced a drastic drop in his Adsense CTR:
The CTR on text ads declined about 60% in the last 2 months with Google’s changes. Image ads on the other hand stayed the same. If you take a screen shot of a text ad and then run it as an image ad it will get 2 times the click thru rate.
As a result total paid click ads dropped for January 2008 and caused the market to react. Google’s shares dropped further to just over $400 (from an all time high of $750+):
With this development, it might scare away huge publishers to move on to more high-return networks. That could further hurt AdSense’s performance and drive down revenues in the coming months.
Still, for most small time publishers, Google will still be king of the hill.