How does the distribution of traffic to the homepages look? One thing search engines do is search for new information, answers to questions and the like. Another thing they do is spell-corrected dns.
Since stackexchange is mostly targeted at the former, I'd expect your numbers to be skewed in interesting ways, which you could partially correct by looking at homepage visits to capture the latter use. (This is a big confounding variable whenever people try to measure comparative statistics between search engines. I've heard that Yahoo and Bing get many more one-word searches than Google, which are likely to be of the spell-corrected dns kind.)
Since stackexchange is mostly targeted at the former, I'd expect your numbers to be skewed in interesting ways, which you could partially correct by looking at homepage visits to capture the latter use. (This is a big confounding variable whenever people try to measure comparative statistics between search engines. I've heard that Yahoo and Bing get many more one-word searches than Google, which are likely to be of the spell-corrected dns kind.)