By now, we’ve all heard the cautionary tales about posting unsavory photos or explosive rants online because they could come back to hurt you—professionally as well as personally.

And with good reason: Companies are increasingly using the Internet to vet potential employees and check up on existing ones. Fifty-five percent of respondents to a 2009 CareerBuilder survey said they used social media in the recruiting process, up from 22 percent in 2008. And Time magazine recently reported that 70 percent of U.S. human resources officers said they rejected job applicants for inappropriate material in online social-networking profiles.

But companies need to use some degree of caution. The Internet gives employers a much better view into the private lives of workers, but that doesn’t mean they can use all of what they find without impunity.

“All it takes to dig up information is a Google search. And virtually all employers are at least using Google. Recruiters use electronic bulletin boards and social-networking sites too. But all of this can potentially raise legal issues,” said Martha J. Zackin, of counsel at Mintz Levin Cohn Ferris Glovsky and Popeo PC.

This might come as a surprise to some employers, Zackin and others said, as some managers are more informed than others about the potential limits regarding their use of what they find online. Some managers have been better educated on the subtleties, nuances, and areas of gray than others, Zackin said.

Lawyers repeatedly point out that the courts have upheld that people who post information online have no expectation of privacy. So companies can search and use most of that information as they see fit. But “most” is not the same as “all.”

The U.S. Equal Employment Opportunity Commission reported that 93,277 workplace-discrimination charges were filed with the federal agency nationwide during fiscal 2009, the second highest level ever, and monetary relief obtained for victims totaled more than $376 million. Attorneys said that suits stemming from the relatively new world of social networking don’t represent the lion's share of all suits filed, but employers as well as employees need to understand what’s acceptable in this new dynamic.

“The advice I give employers is if you can’t ask it, then the fact you found out about it on an Internet page doesn’t mean it should affect your employment decisions,” said C. Forbes Sargent III of Sherin and Lodgen LLP in Boston, Massachusetts.

Employers can certainly verify résumé details or look on Facebook profiles. But if they find information about someone’s religion, marital status, age, disability, or any other protected status and they use that to make an employment decision, then “that’s a potential discriminatory action,” Sargent said.

Similarly, private companies can decline to hire or dismiss workers for posting photos or comments, but they can’t do so if the postings are related to union activities protected by labor laws, said Michael C. Harper, a Boston University Law School professor. He pointed out that the same employment rights and limitations exist in the off-line world too.