While you can connect to the bitcoin network they are currency. If Mt Gox created his own different token system that stopped you from cashing out immediately it would be a virtual currency. The other difference is that Mt Gox would then own this virtual currency,not the person who deposited it. When you buy into a virtual currency it becomes the property of the company.
According to what, though? According to the Mt Gox TOS? What if the TOS said the explicit opposite? Is there case law already regarding this?
Is not the bitcoin casino, in effect, using bitcoin as in-game currency already? If not, what's the differentiating factor?
You are missing the point that virtual currencies are proprietary therefore a bitcoin implementation within a private company would also need to be proprietary. If it is ever used on facebook dont expect it to be used anywhere else by any competitor.
I don't follow the logic here. It doesn't
absolutely have to be the case, just because it is
usually the case. Just because some company says so in their TOS doesn't make it true in every instance, and it doesn't make it true that every company would desire to create their own proprietary virtual currency.