How do you know that crawlers are actually visiting web properties?
We are certain 100% that G crawlers are coming to web properties because of nature of our setup as well as our extensive non-stop in-house testing.
With all the links that we process of our own we have testing URLs, where we post URLs in the mix just in order to get feedback about crawling status.
This is why we are certain that G spiders do visit each and every page we feed them.