I think the story is a bit more complex than what's been suggested.
First, big, well-programmed AMs are still doing well in several markets...WCBS in New York, WBBM in Chicago, WSB in Atlanta, WBZ in Boston, WWJ in Detroit, KMOX in St. Louis, to a lesser extent WINS in New York, KYW in Philadelphia, WBAL in Baltimore and KDKA in Pittsburgh.
Those are not all terrain-challenged markets for FM. What they have in common is that they are big, well-run news or news/talk operations (many owned by CBS).
And given that 8 of the top 10 stations in San Francisco are FM, I'm not sure terrain is that big a factor. KMEL had as good a number 25 years ago as KFRC had 35 years ago. Michael Spears told R&R after KFRC went nostalgia that "we enjoyed spreading the myth that FM didn't work in San Francisco. It wasn't true."
It's as much programming and management that are keeping any AM that used to do strong numbers in SF from doing them today. There are really only five big signals...680, 810, 740, 610 and 560. 810 and 560 are their own thread, and 610 isn't realistically going to get big ratings with Family Radio (which is why CBS sold it to them).
David, do we know how much of KCBS' number comes from AM versus FM?
Pending those numbers, Gregg, AM really hasn't taken a dive in SF, apart from KGO and KSFO, which are way more about Citadel and now Cumulus' management (the same thing is happening to WABC, KABC and WLS) than about being on AM in San Francisco.
960 had numbers they couldn't sell when they were nostalgia. Not much since. And it's been 22 years since 910 had a 1 share or better.
Everybody likes to cite KFRC's sticking with Top 40 well beyond most AM stations, but go back and look at the ratings via David's Americanradiohistory.com site and you'll see that KFRC was on a downward slide from 1979 onward. They'd fallen to 19th place by 1985, the year before they went nostalgia (and rapidly became the #1 AM music station in town again...with demos they couldn't sell). That signal hasn't seen a number above a 1.0 in 22 years (when they came back as oldies in 1993, the FM carried most of the numbers).