Here is a list of 20 top candidate sourcing tools: http://booleanstrings.com/tools/
I am giving a 90-min long webinar on Wednesday, Productivity Tools and Techniques for Internet Sourcing
Additionally, here's a tool to search on Google-plus:Google-Plus Search
Many (most) people have profiles on multiple sites. I always find it amusing when people say "the best talent is on XYZ site" or similar comments. How can that be true when practically everyone is everywhere now?
So, IMHO it may not matter where you look, as much as which site has the best search features to find what you are looking for... Or, which search techniques you are using related to your niche. For that, I would defer to Irina's guidance.
The thing I would keep in mind is that people use each sm tool for different reasons. We keep reading that facebook is best because of its size. But the fact remains that most users are on facebook socializing with friends and family, not to post their professional activities. I know there are reports and articles that say otherwise, but I have not personally seen evidence of that amongst people I know (spanning all demographics).
G+ still seems less active than the other more established sites, but it is widely used in some circles (no pun intended).
LinkedIn still seems to be the obvious default location to search, but there are plenty of people who don't bother joining or maintaining a profile.
Twitter is highly populated, but people seem to use it both personally and professionally.
I totally agree with Kelly. There are tools that offer searches across networks, and the best of those crawl the networks ahead of time. Examples are Zoominfo, ThesocialCV, Talentbin, and Entelo.
Regarding Google+, it can be used to source people who have not joined (limited, but there are some possibilities there).
Here are some custom search engines:
This new tool seems nice, works with LinkedIn http://www.pealk.com/