It's polling. From Kathryn Jean Lopez:

The concept that there is even polling — he has a national approval rating — on Rush Limbaugh seems worth reflecting upon. He’s an unrivaled success in broadcasting and, thus, culture. That’s why they poll on him — wanting a low number (which media coverage of him over the years all but guarantees) to cite to scare people on the Right away from him.

She's on to them.

--Jason Zengerle