I have started lately discovering Content Management Systems, and I was wondering, who is really CMS for?

What I mean by that: is it only for companies, small businesses or individuals, that pays a contractor to make a website that it's users can just upload content through a easy interface.

Or is it used also by programmers, to build their own websites, projects? Would a Facebook, Tweeter, StackExhange ever started by using a CMS, a very powerful one for example.

Would you as a programmer build your own "fancy" website on top of a CMS, for example like Typo3, or you would build it from scratch?

P.S To be more clear is a summary:

What I mean to begin with is, would I as a developer choose a CMS to develop a website that can be scaled with a big base of users, be stuck if I choose to start with a CMS system. What if I build a website using CMS, and the website explodes in popularity, and then I wanted to add much more functionality that I have planed, is it possible that the CMS will limit the growth, because it might have not been build for that kind of scale?