In 1946 we were up to 62 FM radio stations. This fulfilled what was then a plan to allocate only 70 licenses for commercial FM and 20 for non-commercial. Almost all of the 600 FM applications were filed based on the original FM rules, which they had promptly changed. They denied most of them. The Annual report of 1946 clearly notes that 250 FM applications are pending. The start was shaky.
In 1963 docket 14185 adopted most technical FM rules and really established a nation wide tabel of FM assignments. At this time the FCC was openly considering a ban between FM /FM simulcasts as an "a waste of valuable spectrum space." By 1965 at least 7,570,000 homes had FM radios. This leaned heavily toward urban areas. New York City had a penetration of over 50%. There were 4,097 Am stations at that time and 1628 FM stations. You can see the gap closing at least in terms of raw numbers.
The magic year was 1978. That year AM became a ratings minority. Of Arbitron's 500 top 10 stations in the top 50 markets, AM only had 240 stations. To quote from the Broadcasting cable article written that year:
"Take the number-one market, New York, where WBLS, WRFM, WPLJ, and WXLO, all show up in the top 10. Tehre are also four in Chicago (WLOO, WLAK, WBMX, and WCLR) and three in Los Angeles (KJOI, KMET, and KLOS). Rounding out the top-10 markets-Detroit has six, Philadelphia has five Sanfrancisco four, Boston Five Dallas-Ft. Worth six, Pittsburgh six including one AM-FM simulcast combination and Washington eight (including one combo)."That same year there were only 3,896 FM stations to compete with 4,495 AM stations.