This site has only been up for about a year and a half, and traffic has only increased month to month, never decreased. The growth never slowed down, due in part I suspect because of our IAAPA coverage.
On the other hand, GTTP, which has been around for three years now, saw a slight decrease in the winter months this year, most likely because CP wasn't building anything and there wasn't much to talk about. It's only doing about 1,500 people per day right now, where as at the peak of the MF interest it was around twice that.
To answer your question, it depends. A new site might continually grow, while an established one might decrease in the off season.
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Jeff
Webmaster/Admin - CoasterBuzz.com