How are you guys tracking this, going to your references page and clicking the profiles of everybody? If so that will just get you their current state, not necessarily the "state you traded with" if they have moved.
This was my first thought when I read this thread too. My job takes me all over the country, and over the last few years I have sent cards from Michigan, Ohio, Minnesota, Missouri, New Jersey, Maryland, North Carolina, Washington, Colorado, Massachusetts, Florida and probably a few other states I'm not remembering right now.