On Feb 11, 2013 Microsoft announced that so far it has sold 24M Kinect units since it has been introduced in Nov. 2010. The previous sales announcements were:
8M units in the first 60 days of sales (Nov-Dec 2010).
18M in a year from the launch (Jan. 2012, Reuters)
So, it appears to be a significant slowdown in Kinect sales.