Google can't crawl Facebook, Twitter, or most Linkedin data. So the only social "authority" Google has to use, is off G+.
They may not be able to crawl social sites well but they sure as heck can sample a large enough group of people (between the toolbar, Chrome & other things they have access to) to tell what kind of traffic social sites may be driving to other sites - which would be through links, likes and general chatter and twittering. Just because they can't crawl does not mean they can't tell.
They would prefer, I am sure, that they could get that data from G+ but with the amount of information that have at their disposal to see how the web ebbs and flows, I find it hard to believe that they are not using it to tell what people are talking about on social sites.