Chris Havrilla (@havrilla) at Hitachi Consulting shares how she uses social media when searching for candidates.  She talks directly to people who are only using advertised/posted positions:

It is important to note, that finding the right person (or job) may not always happen when there is a posted, open position.  It is a safe bet that there are many potential jobs that go “unadvertised”.  And that number likely grows during times like these as many companies are experiencing budget cuts and may not widely market their jobs.

She also says that these tools are great for research - either before you apply or after to try and network:

If they have a position for which you are sincerely interested and qualified, apply and then use these tools to network your way to an interview.

Chris also goes into details about how she uses LinkedIn, facebook and twitter:

1. LinkedIn - I have found the tool works best for me to facilitate more meaningful contacts with this broader network.  While it may limit the people in my “network” that I can search or view within LinkedIn, I can always use Google to search the public version of LinkedIn profiles.  This gives me access to almost anyone on LinkedIn at any time.
2. Facebook - “I use this tool to establish closer, more meaningful connections with my inner most network.  These are the people you most want to keep up to date with and on – personally and professionally.  Many people are still cautious of using this professionally so, from a direct networking standpoint, it is best to use it for the people you know best.  You can join Groups as a means for more open networking and engaging of others you may not know — and they may be more open to receive.  Lastly, search for target companies and become a “Fan”.  Many companies have created Facebook pages to provide information for or engage with candidates, customers, etc.”
3. Twitter - Give it time – start following some people, watch, lurk, get a feel for it.  Once you have a good group of people you follow that interest you – reply to a few questions or comments – start a dialogue.  “Retweet” things you find interesting, appealing, or valuable.  Then eventually you’ll feel comfortable “tweeting” your own ideas, links, content, etc.