I have notice the traffic drop over the last year or 2 for most positions. One thing you may need to consider is that your site may not always be showing as number 1 for everyone.
The first thing you have to do when checking keyword positions is turn of "customizations based on search activity" on Goggle in the Web History properties. This is a cookie that tells Google to change the serps on you based on what you have clicked on in the past. So if you have clicked on your a site (your own for example) in the next search you do if that site comes up it will appear a lot higher in the search results.
Another thing you have to worry about is localization. Google now displays local results based on the users IP. So what is your number 1 may not be number 1 across the country.