Forty-five percent of the employers surveyed for CareerBuilder.com, the largest US online job site, said they use social networking sites to check on job candidates, up from just 22 percent in a survey conducted last year.
Another 11 percent said they plan to start using social networking sites for screening.
"As social networking grows increasingly pervasive, more employers are utilizing these sites to screen potential employees," CareerBuilder said in a statement.
It said job seekers should "be mindful of the information they post online."
CareerBuilder said that of those who conduct online searches as background checks on job candidates, 29 percent use Facebook, 26 percent use LinkedIn and 21 percent use MySpace.
Eleven percent search blogs while seven percent follow candidates on micro-blogging service Twitter.
Thirty-five percent of those surveyed said they have found content on a social network that caused them not to hire a candidate, CareerBuilder said.
Examples included "provocative or inappropriate photographs or information" or content about drinking or using drugs.
Other reasons cited were badmouthing a previous employer, co-workers or clients, poor communication skills, making discriminatory comments, lying about qualifications or sharing confidential information from a previous employer.
Information found on social networking profiles was not always a negative factor in finding a job.
Eighteen percent of employers said they have found content on social networking sites that caused them to hire the candidate, CareerBuilder said.
Some profiles "provided a good feel for the candidates personality" or supported their professional qualifications while others demonstrated creativity or solid communication skills.
Rosemary Haefner, vice president of human resources at CareerBuilder, recommended that candidates "clean up digital dirt" before beginning a job search by removing photos, content and links which could hurt their chances.
The survey of 2,667 hiring managers and human resource professionals was conducted by Harris Interactive between May 22 and June 10. It has a sampling error of plus or minus 1.9 percentage points.