From what I remember the FM's in the SF Area like in many places really didn't dent the AM's until around the late 70's. I could be wrong but that is what I recall.
The ratings summaries at WorldRadioHistory-dot-com make it really easy to visualize FM's progress in the Bay Area:
Top 10 stations Spring 1976 (total audience 12+):
1. KFRC 8.1
2. KGO 8.0
3. KCBS 6.8
4. KSFO 6.3
5. KFOG-FM 5.4
6. KNBR 4.6
7. KABL (AM) 4.3
8. KNEW 3.6
9. KIOI-FM 3.2
10. KBAY-FM 3.1
(the first place FM makes serious inroads is beautiful music---through KABL-AM would be surprisingly resilient for years to come. KIOI was wildly inconsistent book to book, unable to sustain momentum)
Spring 1977:
1. KGO 7.9
2. KCBS 7.1
3. KFRC 6.6
4. KSFO 5.8
5. KNBR 4.7
6. KFOG-FM 4.5
7. KOIT-FM 4.3
8. KABL (AM) 3.6
9. KNEW 3.2
10. KABL-FM 2.9
(all three FMs in the top ten are beautiful music)
Spring 1978:
1. KFRC 8.4
2. KGO 7.6
3. KSFO 6.5
4. KCBS 5.8
5. KFOG-FM 4.4
6. KIOI-FM 3.9
7. KABL (AM) 3.5
7. KOIT-FM 3.5
9. KSFX-FM 3.4
10.KNBR 3.3
(here we've gone from three FMs in the top 10 to four. KIOI is back and KSFX is having a moment with disco)
Spring 1979:
1. KGO 9.0
2. KFRC 5.8
2. KNBR 5.8
4. KCBS 5.1
5. KFOG-FM 4.1
6. KSOL-FM 3.7
6. KIOI-FM 3.7
6. KOIT-FM 3.7
9. KABL (AM) 3.5
10. KSFO 3.4
(KSOL makes the first real inroad for truly contemporary hit music, as KIOI thrashes about, trying disco for a matter of days, then a heavily-Gold AC)
Spring 1980:
1. KGO 9.0
2. KCBS 5.5
3. KFOG-FM 4.7
4. KFRC 4.4
5. KSOL-FM 4.3
5. KNBR 4.3
7. KYUU-FM 3.6
8. KSFX-FM 3.3
9. KSFO 3.2
10. KDIA 3.1
(KYUU finds a niche playing mainstream hits while KSOL and KSFX are still R&B/dance)
(first FM in the top 3)
Spring 1981:
1. KGO 9.4
2. KSOL-FM 5.6
3. KCBS 5.1
4. KSFO 5.0
5. KFRC 4.8
6. KNBR 4.1
7. KMEL-FM 4.0
7. KYUU-FM 4.0
9. KABL-FM 3.6
10. KIOI-FM 3.3
(first FM in the top 2---half of the top 10 is now FM. KYUU is now the highest-rated mainstream hit station as KFRC decides to defend against KSOL by going heavily rhythmic. KMEL makes the strongest showing for an album rock station in years)
Spring 1982:
1. KGO 8.6
2. KCBS 4.9
3. KSOL-FM 4.8
4. KFRC 4.2
5. KSFO 3.7
5. KMEL-FM 3.7
7. KNBR 3.5
8. KRQR-FM 3.3
9. KBLX-FM 3.2
10. KFOG-FM 3.1
(KMEL and KRQR make it two AORs in the top 10)
Spring 1983:
1. KGO 8.6
2. KCBS 5.1
3. KFRC 4.2
4. KSAN-FM 4.1
5. KNBR 3.6
6. KABL-FM 3.2
6. KSFO 3.2
6. KYUU-FM 3.2
9. KABL (AM) 3.1
9. KIOI-FM 3.1
(Took 'em 2+ years, but KSAN delivers a strong Country number)
Spring 1984:
1. KGO 8.7
2. KSOL-FM 5.1
3. KCBS 4.3
4. KNBR 4.0
5. KYUU-FM 3.6
6. KFRC 3.4
7. KABL-FM 3.0
7. KSAN-FM 3.0
7. KOIT-FM 3.0
7. KSFO 3.0
(Gerry Cagle is out at KFRC. New PD Mike Phillips---from KIOI--- ditches the rhythmic, goes mainstream and loses to KYUU by 0.2)
Spring 1985:
1. KGO 8.2
2. KCBS 5.2
3. KSOL-FM 4.4
4. KYUU-FM 3.8
5. KABL-FM 3.5
6. KNBR 3.4
7. KIOI-FM 3.3
8. KSAN-FM 3.2
9. KABL (AM) 3.1
10. KNEW 2.8
(KFRC and KSFO are gone from the top ten, never to return)
Spring 1986:
1. KGO 7.2
2. KCBS 6.8
3. KNBR 4.9
4. KSOL-FM 4.4
5. KABL-FM 3.7
6. KMEL-FM 3.0
7. KYUU-FM 2.9
7. KBLX-FM 2.9
9. KFOG-FM 2.8
9. KSAN-FM 2.8
...and from this point on, the only AMs that would go top 10 were KGO, KCBS and KNBR. The biggest draws on the AM dial were largely spoken-word (KNBR was morphing to a sports station).