don't you think it would be much better on the center stand.
It doesn't stand for anything. It just means exactly what it says... message board. In this case, an electronic message board. If you are signed in, on the left on of the links will be "My message board," where you can go to respond to people's messages to you.
There is no center stand on this bike, only a side stand.
Short Message Service
PM stands for Personal Message. In Twitter this is a direct message.