On March 7 of 2012, Rush Limbaugh claimed 18,000 advertisers on the approximately 600 radio stations that carry his talk show. I think that his estimate may be reasonable.
At the time, the media was reporting that 28 (national brand name) sponsors had dropped the Rush Limbaugh Show. Limbaugh infamously characterized the hit as "losing a couple of french fries in the container when it's delivered to you at the drive-thru." After more than two decades preaching hate virtually unopposed on the public air waves, Rush could not have known that his brutal attacks on Sandra Fluke had crossed an invisible social line, and that this was just the beginning.
Two radio stations dropped Limbaugh in early March. No one expects additional radio stations to drop his program before 2013. The election insures that many stations won't want to make any dramatic changes prior to the depletion of all that Citizens United election money. Also, a number of contracts between Limbaugh and Cumulus radio stations will expire in 2013. (Cumulus is not revealing the exact date.)
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Here is a thread I posted back in March publicizing Rep. Jackie Speier's reaction to Limbaugh's Fluke comments, containing a simple, one-sentence observation from myself...
We used pressure on advertisers to get Glenn Beck out of mainstream media, why can't we do it with Rush?
...although apparently many in the country were thinking and doing the same thing at the time. I'd like to think that if I helped in any minuscule way at all to have an effect on Limbaugh's power or income, it in some small way made me even just that little much more a better human being.