Very untrue, you need to check your numbers, MS doesn't have a 90% OS share, not even close. OSX holds almost a 20% share and Linux is now up to 11% and climbing steadily. These numbers are also going to change dramatically once Windows 8 is released. While the idea of Windows 8 is a good one it comes too soon. Yes, there is a need to be ahead of the game but MS is messing this one up in a major way by being too far ahead. Windows 8 doesn't have a traditiional interface, it is wholly designed around touch screens. Most people do not have, nor can they afford, touch screens on their PCs yet, it will probably be close to a decade before the average user has them.
Once people see that google+ can do the same things for them that facebook does now and then some they will run to it like rats fleeing a sinking ship. The fact that Facebook has NO customer support isn't going to help their position either.