Forum Moderators: skibum
You may need to give them some incentive to respond -- say a free ebook related to a topic of their interest. You could tell them in advance what the gift is, or indicate it's a surprise gift.
More useful is to create a portrait of what your users are doing. For example, if they're evaluating purchases, that's very useful to know. The only survey I ever did of my visitors, one of the questions was "Have you ever purchased X online?" A surprising number of people answered "yes", and I believe that fact did impress potential advertisers.