Rush Limbaugh once claimed a listenership of 20 million. The method of measuring listenership during that period was the diary – a representative sample of potential radio listeners were asked to record who they listened to. These diaries were typically filled out after the fact. Thus, listeners who liked Rush Limbaugh were prone to exaggerate the amount of time they had listened to Rush.

But the dramatic decrease in actual recorded listenership sampling as noted in the chart reflects the introduction of the Portable People Meter, which is:

"...worn like a pager, and detects hidden audio tones within a station or network's audio stream, logging each time it finds such a signal. It has proved to be much more accurate than the old handwritten logs or wired meters, and is immune to forgetful test subjects."

Portable People Meter, Wikipedia