FWIW, Wikipedia has serious issues with backend scalability: one of the reasons why their history UI is as weird as it is, for example, is that they really really really don't want users using it, due to CPU issues in the diff algorithm (which they now have written in optimized C, and it still causes them problems).

Meanwhile, of course, they are constantly running charity drives to try to get enough money to keep the lights on. I am not certain you'd be able to do the same thing if you were running an open email service instead of managing the worlds seemingly-premier encyclopedia.

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.