Mike,
A well known consultant recently quipped that "According to the experts, AM has been dying ever since TV was invented- and that was 50+ years ago!" In today's world, there are more programming choices than FM signals in most markets, and all the FM'ers want to go for the easy money-ie popular music formats. When one considers that most AM listeners are generally mature, well educated, more affluent, and tend to listen for a reason, usually a specific programming or ideology that the FM's don't provide, it's easy to see why national advertisers spend big bucks on that prime, captive audience. By contrast, if one strictly goes by "ratings", some of the highest rated FM'ers have as 80% of thier audience immature, unemployed kids who have to beg for gas money, if they're even old enough to drive!
I stand by my earlier statement- if a station is making money, regardless of programming choice or rating share, then most would consider it a viable business.