The majority of companies say they use job candidates' Facebook and LinkedIn profiles to evaluate them before extending a job offer, according to a new survey from The Manifest, a B2B news and how-to website.

Meanwhile, more than one-third of companies (39%) evaluate candidates' professional blogs or websites, and 29% look at Twitter profiles.

The Manifest surveyed 505 employees who work in an HR, recruiting, consulting, or executive capacity at their company.

Companies should ensure they are not violating discrimination laws when evaluating candidates on their personal social media profiles.

"My HR advisor strongly encourages us to avoid looking at personal social media accounts when recruiting new hires," said Dr. Terrell Strayhorn, president of Do Good Work Educational Consulting, a consulting firm that conducts and translates academic research. "By looking at a candidate's Facebook page, we might learn something that's less-than-appealing yet irrelevant to the job. Should that influence our hiring decision, it could lead to allegations of employment discrimination."

Some businesses, however, do evaluate social media profiles to guard against potential future issues.

"Social media helps us to be informed about whether employees have posted anything inappropriate such as racist hate speech," said Steve Ozbolt, owner of Emerald City Catering and Events, a catering company in Milwaukee. "That would reflect poorly on our organization."

Companies Use Facebook to Recruit More Than LinkedIn

The survey also found that slightly more companies use Facebook to recruit (70%) than LinkedIn (67%). Companies can use both channels to post jobs and find new candidates.

Slightly more than one-quarter of companies use Twitter (26%) to recruit, and even fewer use YouTube (20%), Reddit (12%), Quora (7%), and Medium (7%).

Both Facebook and LinkedIn have unique recruiting strengths:

Facebook is best for broad, regional searches for less-skilled workers

LinkedIn is best for recruiting specialized, highly skilled workers

Although LinkedIn is thought of by many as the go-to social media channel for recruiting, Facebook has significant benefits.

"Facebook users tend to have much larger networks than on LinkedIn," said Dan Gardo, founder of Element Talent Solutions, an executive recruiting and staffing firm. "When I've used Facebook's job board, posted an opening in a Facebook group for job openings, or shared an open role with my network, I get far more referrals than when I do the same on LinkedIn."