If you work in technology, or are a knowledge worker, it's likely there's a LOT of information and data about you on the internet. And it's likely that smart searchers (human and non-human) can work out what you do and what you are good at. That's one of the reasons that social sourcing tools work so well.
But if you are in retail, or nursing, or education, or a HUGE range of other functions, that might not be true. Sure, you are probably ON the social networks- 67% of online adults are, but that doesn't mean you have any indications of your professional self there. Heck, there's even a trend of software developers hiding their professional information to avoid recruiters!
But as more and more people find professional value on different social sites, this is changing. Whether it's job specific information sharing sites, Google+ groups and hangouts, or simply the increasing perceived value the candidate sees in being "profiles" comprehensively online....for many reasons the number of people and job categories you can find online grows every week.
All of this means that social sourcing is only part of your overall strategy, but it's an essential part, and growing. Just make sure that you integrate it to your other strategies, including employee referrals!
How are you sourcing those "hidden" groups?