Getting More Total Pages in the Google Index

[mahoogle]

We have a four month old site that allows people to create their own content and submit it. Since its launch, we are closing in on 15K pages that could be included in the index. The google "site" command shows 2300 pages in the index. Over the last month it has gone to about 3500 pages in the index and then starts to fall. We recently added a sitemap to see if that will help get more pages in the index.

Is there anything we should be looking for that can cause pages to fall out of the index?

We have some submissions from article sites that are duplicates, but its a smallish percentage of the total body of content. This seems like it could be a flag in regards to quality,

As a note, the pages that are in the index do very well in terms of organic traffic.

Google is also crawling 8K+ pages per day, just not sticking them in the index. Any suggestions to check out would be much appreciated.

[leadegroot]

More backlinks should cause Google to give more weight to your pages and index them.
At the moment its (probably) deciding they just aren't important enough to index.
Encouraging your user-creators to link to their handi work from their blogs and other websites is one place you could start. (An easy encouragement is to make up a bunch of logos with links along the lines of 'see my stuff on example.com')

[mahoogle]

We are seeing pages come in the index, get good traffic and then fall out. It seems to be a revolving door. We would sure like for them to stick in the index.

[leadegroot]

yep, classic signs. More backlinks to some of your lower level pages should help you. :)