About Google's regular crawling of the web
Google's spiders regularly crawl the web to rebuild our index. Crawls are based on many factors such as PageRank, links to a page, and crawling constraints such as the number of parameters in a URL. Any number of factors can affect the crawl frequency of individual sites.
Our crawl process is algorithmic; computer programs determine which sites to crawl, how often, and how many pages to fetch from each site. We don't accept payment to crawl a site more frequently. For tips on maintaining a crawler-friendly website, please visit our Webmaster Guidelines.