Response rates, according to Wikipedia
If you go to the Wikipedia entry for Survey research, specifically the part about Survey methods, you see a brief outline of Telephone, Mail, Online, personal in-home, and personal mall intercept methodologies.
I want you to consider the response rates listed for phone (25%-50%), mail (5%-30%), and online (2%-30%). Do they seem reasonable to you? I hope so, because I entered them into that particular article in the Wikipedia. (In case you don't know already, anyone can edit the Wikipedia -- it's an open-code encyclopedia, an amazing living document. If you haven't used and edited Wikipedia yet, do so today.)
Prior to my edits, an earlier author had pegged the response rates at 40%-60% for phone, and 30%-60% for online. I thought those were unreasonably optimistic and/or grossly outdated, so I changed them. To see the specific changes I made in the Wikipedia, check out this link.
I'd be curious to know what others in the field of survey research think, so please comment.