I've been running my internet radio station, Edgewater Gold Radio for about 17 years now and every single December, listeners drop off. The answer is pretty obvious. The holiday season.
At first I was concerned but once I realized the cause, it made sense to me. During the holidays many people take days off of work but the main reason for an audience drop off is competition from the stations that go all Christmas. These stations gain a substantial audience during this time but this is temporary so don't worry.
As soon as the holidays are over in January, you will most likely see your audience on the rise again! Another audience drop off time is during the Summer but this reduction in audience is not as significant as the drop off in December. Thank goodness it isn't for a long period of time!