Undies has pretty much nailed it.... The survey seems to have been on users rather than admins.

I can say without a doubt that the things I have implemented in the last 6 months to a year to fight spam has had a considerable effect on both the amount of spam that reaches my spam filter and the amount of spam that reaches my users.

I have also noticed, as Undies alludes to, that spam getting to users, including myself, comes in "waves". As the spammers find a new tactic then the spam reaching users increases. The updates to the spam filter or other tactics I employ counter that and the volume that reaches the user becomes reduced. I don't even have to actively monitor it... I can tell something changed when users start telling me "I have begun to get this rubbish in my inbox".... I tell them we are working on it.... In a few days the update or a new tactic/filter we employ when we see a pattern cuts it right back down again.

In short, the volume as a term of pecentage of total doesn't seem to have changed much - on my domains it would still be in the 60-65%+ region.... It's the filters we apply that leave the users with only 10-15% of that spam reaching them.... and they still complain....