I think Google knows about this exact problem. They know that the links people are sharing on Facebook and Twitter have supplanted the site-site links, and are therefore much more important to search quality. To this end, the agreement to include 'real-time' search data from Twitter is partially a misdirection, since the importance of having the data about shared links far exceeds the value of someones 140-character blurb.

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.