Sean Sullivan, a security adviser with F-Secure Labs, said he isn't sure why Facebook isn't simply blocking posts that contain the malicious links. It "seems as if it would be easy," but attackers could be using tricks to evade Facebook's scam filtering systems, he said.

On Wednesday, a Facebook spokesman couldn't say why the links have been so hard to stop, but he did confirm that the company was "in the process of investigating, blocking the links and remediating any affected users."

This type of fake antivirus software has recently become a real problem for Mac users. On Tuesday, Apple began offering daily security updates to its Mac OS X 10.6 operating system. But scammers have become so expert at modifying their programs to evade detection that they have been able to skirt Apple's detection with the latest version of their scam software, Wisniewski said.

The fake software goes by several names, including MacGuard and MacDefender.

The IDG News Service