Security through obscurity

I keep reading that Windows has more security vulnerabilities and exploits simply because it is the most popular OS and that Linux, MacOS and others are not that heavily targeted because they are niche OSs. Here is the latest example of this:

Yes, Microsoft's software has security holes, but hackers mainly love it because there are so many Windows PCs out there. Write one virus and 90% of the wired world could be yours.

Question: considering the enormous negative publicity that a successful exploit against Linux would generate, with the attendant vindication of the OSS detractors and their I-told-you-so's, do you really believe that hackers are not trying their best to discover and exploit security holes in Linux? At the risk of sounding paranoid, I wouldn't put it past Microsoft to invest some of their resources into this; IMO, such a tactic would be more fruitful than hiring the services of hatchet men like ADTI and Laura Didio.