Did this happen to your site too? Here's why it happened.
When you first develop a new site, Google will purposely rank you highly in the interest of exposing you to more customers at first. Also I believe they want you to begin with a good taste in your mouth, so-to-speak, about Google. I have heard this being called a "Cinderella" ranking.
The best thing you can do now is update your content at least twice a week, and start a good external link program. While our site ranks 400,000 worldwide (our main competitors rank 8-6 million), the site must age a bit before Google instills more trust in it. Of course I miss the old site being ranked 1st in organic results, but sometimes innovation requires sacrifice, or something to that effect. To your success!