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OSE only does crawl a portion of the web. We are trying to crawl more URLs, but also make sure our data is fresh. It's a bit of a massive undertaking to try to amass data at this scale. Check out the blogpost for our May update at http://www.seomoz.org/blog/may-mozscape-index-update164-billion-urls for some more information about OSE, where we're at, and where we're going.