I'm going to call BS on this one.
The population of Tibet is only 2.91 million.
http://en.wikipedia.org/wiki/Tibet_Autonomous_Region#Demogra...
Are you saying they "disappeared" 1/3 of the population?
Do not make the mistake of thinking that anything is too monstrous for a national government, particularly one that cannot be held to account by anyone except a strong alliance of other national governments: China, Russia, or the U.S.
The population of "Tibet" includes Han occupiers. It is entirely possible that someone has "vanished" more than 1/3 of Zang (Tibetan) in that region. But some Zang also live outside the nominal political borders of Tibet. And due to forced displacement, and the manner in which Beijing handles China's ethnic minorities, like Zang and Uighurs, or even Han peasants, it is also possible that the "missing" people are still alive, but shoved into a bureaucratic black hole that prevents them from communicating with anyone connected to the rest of the world.