Thanks for the responses. Google Analytics works for the .html portion, but not the individual map services. Fiddler only tracks from the client side, not from the server side.
The geocortex-optimizer looks promising.
I also learned that if you have admin rights to your ArcGIS Server, you can get usage stats on the map services by making a GIS Server connection, Manage Services type, and then right clicking on that connection. It seems that the geocortex optimizer is probably pulling the same data as this stats display. The log file also records when services are called, but I'm not sure if it records when they are turned off.