Probing into prospective employees’ social networking sites is becoming an ever more common practice among employers, according to a new survey commissioned by CareerBuilder. Forty-five percent of the 2,667 hiring managers surveyed reported checking up on candidates’ Facebook or MySpace profiles–more than twice the 22 percent who copped to snooping last year.

For 35 percent of the surveyed employers, the information they found kept them from hiring a candidate. The most common information negatively influencing the decision included tasteless pictures or messages, indications the candidate consumed alcohol or used drugs, and nasty comments about previous employers.

While it’s reasonable to look into inappropriate behaviors publicly displayed online, employers need to protect themselves from potential discrimination suits.

“Employers have to defend themselves by having legitimate reasons for denying the application, whether it’s more qualified applicants or another legitimate basis for the action,” says Gregory Iskander, of counsel at Littler Mendelson.