Those are massively read (as opposed to write) oriented, the content is the same for everybody, and as such cachable. Now, think something like Gmail, Basecamp

Those are massively read (as opposed to write) oriented, the content is the same for everybody, and as such cachable. Now, think something like Gmail, Basecamp, Google Docs, Facebook where that is not the case, and we're talking about an order of magnitude if not more expensive and powerful infrastructure needed.