Facebook has been the victim of five different security problems in the past week, says Trend Micro.

According to the security firm, four hoax applications have become available on the social networking site along with a new variation of the Koobface virus. The latest version of Koobface, which was first detected at the end of last year, directs users to a fake YouTube page where they are encouraged to install malware.

Two of the hoax applications that have been downloaded by Facebook users include 'F a c e b o o k - closing down!!!' and 'Error Check System'. By downloading the app, users give hackers access to their profile and personal information, and also unwittingly forwarding fake messages to their friends encouraging them to download the program.

Rik Ferguson, senior security advisor at Trend Micro, told the BBC: "It's been a pretty bad week for social networking in general".

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"It's almost as if the applications we have seen this week are a proof-of-concept," he said. "It would be much better for them to generate rogue applications that did not look like rogue applications."

Ferguson also revealed that he believes hackers are currently working on creating apps that don't initially appear to be malicious. He predicts these apps will appear on social networking sites very soon.

"One of the problems is that Facebook allows anybody to write an application and third party applications are not vetted before they are released to the public. Even as Facebook stamps out one malignant application, it can pop up in another place," said Graham Cluley, senior technology consultant at Sophos

However, it appears Facebook is still refusing to vet apps before they are made available on the site.

Founder Mark Zuckerberg told Radio One last month: "Our philosophy is that having an open system anyone can participate in is generally better".

See also: Five Facebook scams & how to avoid them