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I recently had such a spike and now in the following days with normal visitor numbers my eCPM is terrible. It is so low I am even considering dropping Adsense and to make my own ads for affiliate programs.
Do other webmasters with Adsense and spikes in visitor numbers have this same experience of very low eCPM when visitor numbers are back to normal? Trying to determine if it is coincidence or a pattern.
I know I've dropped some of my AdWords campaigns and lowered some PPC amounts because of poor performance since the change. Maybe it's just August but we'll see.