The computer worm responsible for the biggest attack in years has infected at least 1 out of every 16 PCs worldwide, a security company said Wednesday, and may have managed to compromise as many as nearly 1 in 3.

According to Panda Security, almost 6% of the Windows systems scanned with its antivirus technology were found to be infected with “Downadup,” a worm that began aggressive attacks just over a week ago. Panda was one of the first security firms to ring the alarm over Downadup, when it raised its security threat level Jan. 12 as reports of attacks mounted.

Using data from antivirus scans performed by its consumer-grade security software, and by a free online scanning tool it makes available on its Web site, Panda found 111,379 PCs infected with the worm out of a pool of 2 million machines.

In the social networking arms race, without a doubt Facebook and Myspace are the leading contenders. But now, at least on a global stage, it looks like a definitive leader between the two giants has emerged, with Facebook claiming double the visitors of Myspace.

Reported by TechCrunch, according to a new data report from comScore Facebook drew more than 200 million unique visitors in the month of December compared to Myspace’s number, just over 100 million. Just six months ago Facebook and Myspace were tied around 100 million visitors, but Facebook’s numbers shot up at a rate of 10.8 percent per month on the global scale while Myspace stayed stagnant.

In the U.S., Myspace still leads, but has not grown its visitor numbers for most of the year. Facebook, on the other hand, has been increasing at a rate around 3.8 percent each month so while Myspace is currently the leader, it won’t be for long if the trend continues. By the end of this year, Facebook should have a lead over Myspace in the U.S., in addition to absolutely trouncing Myspace worldwide.