A related point: whoever first owns the data from all link aggregators (digg, reddit, mixx, etc) and all URL shorteners (bit.ly, tinyurl, ad nauseum), and weighs those results more heavily is going to have an awesome search engine... albeit better for entertainment than productivity.
Except I'd never find the obscure stuff that Google helps me find every day. I don't think a lot of links to manuals,mailing lists, etc show up on any of the sources you mention.