It happened because Windows is an insecure piece of junk. Anyone who knows anything about security knows that this kind of disaster was only a matter of time. Windows botnets are responsible for DDoS attacks and most of e-mail spam. You cannot secure Windows. Microsoft keeps saying that they will, and they always fail. Period.
Twitter and Facebook users are just the kind of people M$ can't afford to lose. They are the decision makers who decided Vista's fate and will soon do the same for Windows 7. They are generally aware of Windows performance problems that 7 is still slower than XP and that Ubuntu is faster than both. Will people finally heed the security warning and walk away from Windows at work?"