A search engine doesn’t need to rely on powerful algorithms if it has enough people classifying sites for them. If you bother to bookmark a Web page in your browser, it means you think it's worth coming back to. When you store your bookmarks in Xmarks (previously Foxmarks) to synchronize them between different machines, the service takes note and scores popular sites based on how many people bookmark them. With 600 million bookmarks, that’s enough information to give a reasonably accurate rating and to do some more analysis.