Emboldened by Rush Limbaugh’s public apology over the weekend to a law school student whom he had called a “slut” and a “prostitute,” critics of the radio talk show host are intensifying their online campaign against his advertisers. The apology, they said, was a signal that the campaign was working...Mr. Limbaugh, who is estimated to make $50 million a year and whose program is a profit center for Premiere Radio Networks, the company that syndicates it...Premiere’s parent company, Clear Channel, deferred questions to Premiere, which declined to answer questions about the effect of the ad boycott or the widespread anger at Mr. Limbaugh...In a statement, Premiere — best known for conservative talk shows — said it was committed to giving listeners access to a broad range of opinion and commentary.