These days, almost everything you put is online. And some companies are taking advantage of this to go over social media and find more information about candidates than they could cover with just a job interview.

But not all companies are doing this. A 2011 survey from the Society of Human Resource Management showed that just 18 percent of companies used social networking websites to screen job candidates, and more recent numbers have increased that to just two in five. Given the ubiquity of social media, why aren’t more companies taking advantage of it to uncover more information?

The answer is that social media screening is not as useful as you would think, and in the worst case can be dangerous from a human resources and legal perspective. If you intend to use social media to vet potential applicants, you need to seriously understand what you can and cannot do when looking at their backgrounds.

First, your company needs to have clear policies in place on what they are looking for when inspecting an applicant’s social media posts. A company will generally check for three things. The first is to see whether there is anything on a candidate’s social media profile which contradicts his resume and interview statements. The second is to see whether the candidate has a history of badmouthing his employees. And the third is the candidate’s character, whether it is a history of discriminatory statements or one of drug or alcohol abuse. This third category appears to be the most popular reason to use social media.

But while it may be the most popular, it is also the most problematic, especially for vetting grad law jobs. First, there is the fact that this can be largely up to the employer’s interpretation. Just because a candidate has a photo of himself with a can of beer does not mean he is a degenerate drunken party animal. In fact, it could mean that he is social and outgoing. Using social media to check character just brings more subjectivity in the hiring process.

That subjectivity is bad because of the risk of uncovering protected information. If you check a candidate’s social media profile, you will also learn his age, religion, race, and other protected classes he might be in as well. Since you cannot unlearn such information, it increases the risk that your company will be open to a discrimination lawsuit upon denying a candidate the position.

One example of this occurred in 2007, when the University of Kentucky denied an astronomer position to astrophysicist Martin Gaskell based on Internet posts which showed that the candidate was an evangelical Christian and had made controversial remarks on evolution. Gaskell sued the University afterwards, and the University eventually settled and paid $125,000.

And while candidates generally understand that you will be conducting a social media search on them, you should be upfront about it. Have a clear policy in place which dictates the social media websites you will check, and stick to LinkedIn, Facebook, and perhaps Twitter.

But under no circumstances should you ever ask for a candidate’s social media password. Several states have passed laws banning such practices, including California and Illinois. And even in states where it is legal, the potential information you might uncover is not worth losing candidates who will not trust a company which commits such practices.

Using social media can be useful in ensuring that a candidate’s background is really what he claims it is, but using it as a tool to vet their moral character is not a wise move. Employers need to understand that while social media vetting can be useful, it poses serious risks and a company may be better off not using it at all.