That's an intriguing idea, assuming that the likes of Facebook and Twitter would want to share that data with Google and would be able to do so without running into privacy concerns given that the links were presumably not public if Google couldn't see them in the first place.

I've been wondering whether something like this might explain how one of my sites, which has about two incoming public links in the entire universe and probably a page rank of 0 but seems to attract a decent level of FB/Twitter interest, hits the first SERP at least once for numerous queries in its niche.