Linux is a good example of this. Because of linux's fast adoption, more security flaws are being found.
I agree, in that this will hold true with any product as its usage increases. However, if a vulnerability is found in one product by the security community, competing products are also usually tested to see if they also are vulnerable, and reports made accordingly.
With this in mind, there are, on average, 20 unfixed vulnerabilities in the most up-to-date, patched version of IE. 6, on average are "critical" vulnerabilities. Compare to competing web browsers, say Mozilla/derivatives and Opera, where there is on average one unfixed vulnerability at any time.
I said previously about MS prioritising functionality far over security, reliability and performance. I'll add to that now. What is far more dangerous about MS software is that by default, the state in which the product is shipped to the customer, is with everything wide open. On other operating systems, for example - ssh on UNIX derivatives, you can't log in remotely as the 'root' (highest privileges) user by default. It has to be configured to do that.
When linux is a huge monopolly
Unless some company chooses to adopt MS typical tactics (which should be illegal), no operating system will ever have a monopoly. Why? Because users will always have a choice. The opposite is a monopoly situation, when users have little or no choice.