I did hear a second-hand story about Yahoo web search, for what it's worth. When the Delicious acquisition happened, of course they were super excited for this very reason -- that Delicious should constitute a high-quality dataset (that Google didn't have!) Then they tried all sorts of things to incorporate it into their relevance ranking algorithm, but never could get it to work. I personally think Yahoo search had good relevance algorithm people, and therefore, if you believe this story, that Delicious data is not useful for web search relevance.
So I understand this story might be hard for others to verify. But I think it's reasonable to assume that if the data really was valuable, they would have used it for web search, and someone would have bragged about it at some point -- especially given the large amount of interest from the wider developer and computer science community about how potentially useful the Delicious dataset should be.