According to TVbytheNumbers.com, "It's the Great Pumpkin, Sam Winchester" had 1.97 million viewers.
Still from the site, "Speaking of Supernatural, did you ever notice how at least in the aggregate, the Thursday CW shows do better in repeats than some of the shows targeted at women 18-34 do with original episodes?" (source).
Supernatural will return with new episodes on March 12, 2009.