Is it not true that WOR-FM began doing this in the mid to late 60's? I could be wrong date-wise, though.
Group One Broadcasting in Dayton, Ohio began "experimenting" with an FM Top 40 format in 1969, after flipping its AM station, WONE (yes, same call letters as the FM in Akron) to Country.
Originally, the station was run automated under PD Rick Stevens. He was told by the managers that their goal was to "cut into" the teen market dominated at the time by WING-AM, but not become so popular that the FM overtook, in terms of market and share, its sister country AM.
When Stevens departed, the programming reins were handed over the a man known in the Denver area well...Bill Struck. One by one, as billing improved, Struck added new live DJ's. At first, the shifts were 6 hours long, but eventually, a 6th full timer was hired, and shorter shifts were arranged. (The station had 2 overnight jocks initially, who both jocked most of the weekend shifts).
About this time, (around late 1970 or early '71) the station, at 104.7 Mhz renamed itself "WTUE". From there until August, 1975, WTUE was a formidable standalone Top 40 station, which eventually, tied WING in Women, 18-34...and was #1 with teens.
When the Top 40 format deflated by the mid-70's, the station flipped to AOR...and later Classic Rock. But, WTUE was certainly one of the first standalone Top 40 stations in America.
Of course, one could also argue that Dayton also had WDAO-FM at 107.7 in 1964. First programmed by Dayton's legendary Gene "By Golly" Barry (who made the original suggestion for the format change), WDAO was an Urban FM which made it a point to play a lot of pop crossover records for the time. The station probably rightfully should be considered an R & B or "Soul" station for its time, but the crossover 'DAO played at the time drew a good share of white teens to the station as well. And the jocks? Uptempo...top 40 jocks in every respect of the word. WDAO stayed as part of the radio landscape on FM till the early 80's, though it still continues today at 1210 AM (the frequency owned by WDAO's former sister station, WAVI.)
This probably means that Dayton, Ohio had far greater FM penetration than most markets by the late 60's/early 70's. Dayton probably had more teens listening to FM back then than you'd have seen in a lot of markets.