In 1988, AM radio was a dying entity. With a few exceptions (most notably, Nashville's WSM-AM, home of the Grand Ole Opry), AM radio was relegated to the formats that FM did not want: religious programming, foreign languages, old country, R&B, and pop crooners.
Then on August 1, 1988, the Rush Limbaugh program began, airing on AM radio. The show took off, and in the process revived AM radio as a vibrant, legitimate outlet.
Love him or hate him (it's very difficult, if not impossible, to find anyone in the middle when it comes to the conservative host), Rush Limbaugh's popularity elevated talk radio from a mostly local outlet to a nation, vital necessity. His show is now carried by over 600 radio stations with an audience of 20 million. Before Limbaugh came along, six radio stations and an audience of 20 thousand for a talk show was unthinkable. Now the format is so popular that it's practically all you can find on AM radio.
Happy anniversary to the Rush Limbaugh program!