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Try getting links for important pages. As for google crawling and indexing: i have websites that contain 10k of URL's on which i know, half of that is not relevant for search at all. Google does it completely automaticly and cuts out the most duplicate or non-relevant pages in comparison to what i offer in my sitemap and brings 4k of pages or so in actual search. It's really no issue. You cant tell me you got 20 million of unique (user) generated content.