Scalability (Design) Considerations

I tried searching for accurate information, but I fail to find any. So I am asking you veterans here.

Firstly, What kind of requests has a platform like Facebook/stakoverflow has to perform per unit of time?

Secondly, What kind of resources and restrictions are required to run such a web services, let's say for 10 million users? (hardware/software considerations)

I know my question is a bit vague, but I am having a hard time understanding the scalability in such an application.