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They will crawl anyone who pays.
They selectively (but widely) crawl the net for free.
The criteria for the free crawl are not totally clear. They obviously crawl "important" sites (I suspect that links play a part in that definition), but also crawl fairly obscure sites.
My main site, which has a fair number of links pointing to it, was crawled as soon as they started their new approach. A new site I started this past spring, with very few links, had its index page indexed almost immediately, but the interior pages only started being indexed a month or so ago.
There are three routes into Ink:
1. PFI
2. Use a free submit link
3. Wait for an Ink spider to find a link to your site