Well, as Laurie Sullivan (writer and editor for MediaPost) points out in her column yesterday -"Click-Through Ad Rates Level Off" that now...
".......the industry has found a balance where the growth in the number of impressions does not outpace users' growth in clicks.
(And that's according to Ariel Geifman the principal research analyst at MediaMind and is contained within a Report, conducted by MediaMind.)
"....Evidence shows that the success of online advertising has oddly been the prime cause for the decline in CTR performance, according to the report...... As more budgets poured into display, [then] users were exposed to more and more ads. The number of ads a user clicked on did not catch up with the number of ads users viewed, so it reduced the overall CTR."
That makes sense, surely?
LCDisclosure: Long LOOK and happy to be so, too...