He argues the search reduces the risk that employers may confuse the job candidate with someone else or expose the company to information that is not legally allowable or relevant. “Googling someone is ridiculously unfair,” he said. “An employer could discriminate against someone inadvertently. Or worse, they are exposing themselves to all kinds of allegations about discrimination.”

Marc S. Rotenberg, president of the Electronic Privacy Information Center, based in Washington, said that employers were entitled to gather information to make a determination about job-related expertise, but he expressed concern that “employers should not be judging what people in their private lives do away from the workplace.”

Less than a third of the data surfaced by Mr. Drucker’s firm comes from such major social platforms as Facebook, Twitter and MySpace. He said much of the negative information about job candidates comes from deep Web searches that find comments on blogs and posts on smaller social sites, like Tumblr, the blogging site, as well as Yahoo user groups, e-commerce sites, bulletin boards and even Craigslist.

Then there are the photos and videos that people post — or find themselves tagged in — on Facebook and YouTube and other sharing sites like Flickr, Picasa, Yfrog and Photobucket.

And it is photos and videos that seem to get most people in trouble. “Sexually explicit photos and videos are beyond comprehension,” Mr. Drucker said. “We also see flagrant displays of weapons. And we see a lot of illegal activity. Lots and lots of pictures of drug use.”

He recalled one man who had 15 pages of photos showing himself with various guns, including an assault rifle. Another man included pictures of himself standing in a greenhouse with large marijuana plants.

Given complex “terms of service” agreements on most sites and Web applications, Mr. Rotenberg said people do not always realize that comments or content they generate are publicly available.