Hotmail seeing a slowdown in traffic also

I have noted that in the past 5 months, the total amount of mail that we are seeing in the Exchange Hosted Services network is decreasing slowly but steadily.  I wasn't sure if this was a common trend (since the antispam articles I read all say spam is on the rise) or if it was localized in my neck of the woods.

I checked some Hotmail statistics today and it is the same over there; the trend is not as noticeable but even they are seeing a gradual slowdown in the amount of traffic they filter.  I wonder if Yahoo or Gmail are seeing the same thing?

Let's assume for a minute that the trend is true and that it is not localized to Microsoft.  What could be causing this slowdown in spam traffic?  Less profitability from people not paying anymore?  Virus writers not being as successful in getting products deployed (ie, better anti-viruses)?  China clamping down on spammers?  Something else?