It's simple; Zillow acquired Mortech and Hotpads at the end of 2012 and would now include the monthly users from these sites in its combined Alexa ranking. That in itself could be hiding a serious lack of growth for Zillow itself, and may explain why they rushed the acquisitions the way they did. Just my 2 cents. Until Zillow publish the user details on February 13th there will continue to be speculation how good or bad their performance is.
So a stat is only significant when it supports your case (an extremely weak case in this situation)? You were the one that introduced the Alexa ranking to the discussion. How much did you lose shorting based on a seasonal data point?