Well websites like Youtube/Google are hosted by Google+Youtube as it states it and is pretty known that no host could handle the resources they use daily.. Google I know for sure has a whole room, As I read something a while back.
For Facebook it may be the same thing hosted by themselves can always check out
Google search: who hosts a website and click the first few links.
Well, I know most of the biggest websites on the Internet host themselves, due to the amount of resources they use.
They'd need hundreds of dedicated servers for everything. Do you think a normal host could provide THAT to a "single customer"?