To be fair, I have never had any malware with OSX and I'm certain I will not. OSX by its roots (BSD) means it doesn't get the kind of malware that plagues all those M$ Windows computers. I feel safe with OSX and have no need for antivirus. If you give our your root password to a random program, well, you're stupid. But if you use Windows you get infected just by connecting to the internet. I've never had such experience with my Mac.
I don't think Linux developers understand the problem. Once again. It's not about the technology underhood or that the protocol is open. The fact is these things need to be able to call to the "real world" and be able to receive calls from there. Basement geeks probably don't understand it, but that's what most normal people use Skype for. It will also need clients on tons of mobile phones AND it needs to be able to be used with Skype users. Now that Microsoft owns part of Facebook they will probably start using Skype too. You won't win this just because your application is "open".