I searched around on the web, and here and was not very successful at getting an idea of how much GA under-reports traffic. Does anyone know?
We're running some ad campaigns from FB, StumbleUpon and others, all tagged with GA tracking tags. FB reports about 50% more click-throughs than we're seeing in GA. Of course GA is going to under-report for users with cookies or JS disabled, or adblock, etc. But this 50% seems pretty high to me.
Any pointers to reasonably accurate estimates of this would be greatly appreciated.