From CNN.com:
Not only are valuable connections forged with potential employers and colleagues on sites like Facebook, Myspace, Twitter and LinkedIn, but openings are also posted there, sometimes in lieu of job boards.
If you fan a company on Facebook or follow internal hiring managers on Twitter, you might be the first to find out about job openings at the employer of your choice.
“Social network is going to take over job boards,” he said. The greatest advantage to Facebook, Twitter, Myspace and LinkedIn is that job candidates and employers can meet through people. Those connections make it easier to break the ice, he said.
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