The security firm found that a quarter of companies have fallen victim to spam, phishing or malware attacks via these sites.
Graham Cluley, senior technology consultant at Sophos, pointed out that using the corporate contact details available on LinkedIn, for example, a phishing attack could easily be set up to mine intranet log-in or user account details from new employees.
"If your users are sharing too much online, it can give away clues regarding corporate security, not just personal identity information," he said. " Companies need to educate their staff."
Businesses should also install web security which can scan content in real time to prevent users straying to malicious sites, he added.
Social networking sites like Facebook have generally been proactive in raising awareness about security risks, but they "could do better" to protect their users from malware, spam or phishing attacks, argued Cluley.
"I'd like to see social networking sites understand that they've got a huge amount of traffic travelling through their users, so they should be more proactive in scanning it," he said.