There are a lot of studies around about "the best sources of hire"....and sourcers themselves don't seem to feature much more than a footnote in any of them. But there are many, many great sourcers out there hunting down great candidates.
A sourcer might find a candidate using incredible boolean sourcing, an automated sourcing tool, or through engaging with prospective candidate pools in places like Meetup or LinkedIn (or through a thousand other techniques!). Then they'll talk to the candidate, maybe get them in a talent community or hackerthon, educate them on the company, get to know the candidate, and generally warm them up until they are really interested.
After all that high touch, high value work, BINGO! The sourcer gets the candidate to apply for a job, probably through a link to the corporate career site. And in the vast majority of sourcing/recruiting analytics, the "source of hire" then becomes that career site, not the human being who did all the work. From a data perspective, the work of the sourcer is lost...and so the value of the sourcer's work is left to "gut feel" instead of data.
There are some sourcers who use clever techniques with custom url's and other goodness to overcome this, but probably not enough of them! And some organizations and sourcing analytics tools (like ours) track the granular detail of who did what to attract the candidate, but definitely not enough of them!
Before you can really understand the best sources of hire, this level of detail needs to be tracked, and tracked by job function (since all recruiting and sourcing efforts should be tracked by job function!). As a sourcer, how are YOU making sure your work gets included in the analytics of best source of hire? I mean, aren't you an excellent source of hire yourself?