If I recall correctly, Yahoo tested using this data a few years ago and found the signals not to be as useful as others they used at the time.
Of course, Bing and Google have been working to include more social signals in rankings: http://searchengineland.com/what-social-signals-do-google-bi...
The way I see it, search engines are entirely inclusive, while bookmarking sites are selectively inclusive. For certain ultra-specific queries, I'd much prefer the search engine approach -- I want as much breadth as possible. For other not-so-specific queries, I'd much prefer the result sites to be vouched for by people. Looking one step further, probably for a decent chunked size of my search queries, I'm OK if the number of sites it is searching over is only about 10,000,000 (my estimate for how many sites on Delicious have been bookmarked by more than 20 users).
What I find exciting is that the two can be effectively combined. Take all the normal results that a search engine would show, but give it a boost based on the log of how many times it's been bookmarked. It really is that simple.