I can tell you from experience that once you get over a certain traffic/load threshold, you need multiple dedicated servers... at the time I did it, it was several million hits per day. When that gets too big for it’s britches, you need to pay for mega-bandwidth and load balancing. And when that gets untenable, you need traffic distribution balanced to regional data centers so that the whole internet isn’t rushing billions of requests to a single data center.
... but then, I’m sure that would be a good problem to have.
I still say, even though facebook unseated myspace, I don’t think anything will unseat facebook until there is a paradigm shift in the technology sets (browser/app combo)