The summary from eMarketer ("Job Candidates Both Hurt and Helped by Social Networks"), is worth reading (click on the citation link below) - the full article has additional stats/charts.
Blurring the line between professional and private
Most social network users have heard of the consequences of having an unprofessional profile page, and know potential employers might look them up online.
Respondents were most likely to use search engines such as Yahoo! or Google to check out job candidates online (41%), followed by Facebook (29%) and LinkedIn (26%).
The findings were more likely to get candidates rejected than hired: 35% of HR professionals said social networking content had caused them to eliminate a candidate, while only 18% reported deciding to employ someone based on a profile.