Conversation so far:
What did Casey Kasem have to do with the American Top 40?
Kasem, along with Don Bustany, Tom Rounds, and Ron Jacobs, launched the weekly radio program American Top 40 (AT40).
When did they launch the program?
On July 4, 1970.
What else happened during this time period?
Loosely based on the TV program Your Hit Parade, the show counted down from #40 on the pop charts to #1-.
How long did the show run?
In October 1978, the show expanded from three hours a week to four.
American Top 40's success spawned several imitators including a weekly half-hour music.
How many viewers/listeners did the show have?
CANNOTANSWER.

Question: What else is interesting about the Top 40?
Self-contained rewrite: Besides expanding from three hours a week to four, what else is interesting about the Top 40?