I heard that Bethesda will have its own platform: Bethesda.net. I am just wondering, if I bought a copy of disc of Fallout 4 on amazon, can I still activate it on steam just like Fallout 3 and Fallout New Vegas, or must I activate it on Bethesda's own platform?

Very little is known about Bethesda.net at the moment, but I think you should be fine activating it on Steam. Bethesda.net seems similar to Nexus rather than Steam or Origin.