Emergence of various vertical search engines highlights the fact that a
single ranking technology cannot deal with the complexity and scale of search
problems. For example, technology behind video and image search is very
different from general web search. Their ranking functions share few features.
Question answering websites (e.g., Yahoo! Answer) can make use of text matching
and click features developed for general web, but they have unique page
structures and rich user feedback, e.g., thumbs up and thumbs down ratings in
Yahoo! answer, which greatly benefit their own ranking.