Nearly two thirds (60 percent) of businesses believe social network Facebook poses the biggest threat to security, says Sophos.

Research by the security firm for its Security Threat 2010 report revealed that MySpace was voted second with 18 percent, closely followed by Twitter with 17 percent of votes.

Only 4 percent named LinkedIn as a security threat.

"Facebook is by far the largest social network - and you'll find more bad apples in the biggest orchard," said Graham Cluely from Sophos in a blog.

Cluely added that the security team at Facebook works hard to counter threats on their site - it's just that policing 350 million users can't be an easy job for anyone.

However, he pointed out that when Facebook rolled-out its new recommended privacy settings late last year, it was "a backwards step, encouraging many users to share their information with everybody on the internet".

He also warned users that while LinkedIn is considered to be the lest threatening social network, it still provides hackers with "a sizeable pool of information".