Well maybe that's because Macs are small potatoes compared to Windows and Windows are designed for small to large businesses as well as Linux. A lot of hackers like to exploit and create damage to anything that generate profits. I rarely see big business places using Macs as any kind of servers or anything, except for video editing and computer graphics. That's because that's what Macs are really designed for, not designed to do business. That's what IBM is for and a lot of companies made IBM PC clones and Windows eventually made its way on these IBM machines as well as the clones. A lot of hackers don't care about Macs cuz like I said, probably because Macs are small potatoes and they don't want to waste their time hacking into someone's computer and find some porn, computer graphic pictures or games. They want to break into computers that contain private, exclusive data that the world, except for the user, shouldn't see. Linux is an open source operating system and hackers love to manipulate data so they take advantage of the open source window and try to insert some kind of malicious code in the OS so it will run whatever the hacker has designed it to.
As for "16 million users" using Macs, I'm not quite surprised because Windows are designed for business use, Macs are designed for home use because a lot of people around the world are computer illiterate and they don't know how to use a computer, let alone Windows. That's why Mac came into the picture, to help ease the worry and the agony of using a computer because Apple has made it simple and easy for a dumbfounded user to use and be happy.
Any one of you who don't like the previous version of Windows, including the XP and the upcoming Windows Vista, here's a simple solution: Don't use Windows based computers, stick to the Macs so you don't have to complain. I'm not using a Mac at all so you don't see me complaining 
