"After they’ve conducted such a search, more than one-third (35%) of employers report they have found content on social networking sites that caused them not to hire the candidate, CareerBuilder said.Nearly half of all U.S. employser use social networking sites to investigate candidates
...Of those who conduct online searches/background checks of job candidates, 29% use Facebook, 26% use LinkedIn and 21% use MySpace, the survey found. One-in-ten (11% ) search blogs, while 7% follow candidates on Twitter.
Social media can be a positive influence in the job search, CareerBuilder said, noting that in many cases job seekers are encouraged to leverage social media when advertising their skills and experience. To this point, 18% of employers report finding content on social networking sites that caused them to hire a candidate."
Be careful of any job offers that require an "investment" to get started. To help prevent identity theft, do your research to make sure a real job offer really exists before you share your Social Security number.