When your site launched, did you have a massive PR blitz then stop? This could seriously affect your SERPs due the spike in link acquisition. Rather than the blitz, try following Rand’s staged link acquisition strategy for a natural link-growth curve the engines will continue to index.
After the launch, you can rely on your network effects for a bit. Give users a reason to link to your site – user generated content, badges, great profiles, etc. When the link acquisition rate begins to go down, generate some linkbait to prop up the link acquisition rate. This will help you stay in the index in the long term.