McAfee has released their quarterly threat report for the fourth quarter of 2011, and some of the results may surprise you. The report examines the biggest threats to online security for both the fourth quarter and all of 2011. The report shows that while spam email seems to be on the decline, malware remains a significant threat.

While malware remains a significant problem on computers – PCs far moreso than Macs – the biggest area of growth for malware has turned out to be mobile. Google’s Android platform is increasingly the target of malware. According to the report, mobile malware and adware were at an all-time high in 2011 in general, and in the fourth quarter in particular. Most Android malware hijacks users’ phones and uses them to send SMS messages. The ease with with an Android device can be rooted makes the problem worse, as the same exploits rooters use to hack the operating system to increase their ability to customize it can also be used by attackers to hijack the phones.

Meanwhile, malware growth on PCs appears to have declined in the fourth quarter. Yet the study warns that there were still over 75 million unique malware samples recorded, meaning that users cannot afford to relax their vigilance. Macs remain the most secure platform. Despite a spike in June, the malware threat on Mac computers remains all but nonexistent. Nevertheless, the report reminds users that “it’s always wise to protect your system, even if it’s a MacBook Air.”

Perhaps the best news to come out of the report involves spam. While spam email messages still vastly outnumber legitimate email messages, the gap appears to be closing. While some countries – including the United States – experienced slight increases in spam volume, overall volume was the lowest it’s been in years. The report cautions, though, that the threats represented by spam email remain as strong as ever, and perhaps even stronger. While volume has certainly decreased, the report notes that address lists for spambots have become more accurate and more precisely targeted.

The full report, which includes a more detailed breakdown of threats and a country-by-country breakdown of spam rates, can be found in PDF form here. Another interesting detail tucked away in the report’s discussion of malware is that the majority of malicious sites are actually hosted in the United States.

Have you noticed a decline in the amount of spam you’ve been getting? If you’re an Android user, have you had an problems with malware? Let us know in the comments?