I think part of the difference is due to the fact that the business of administering healthcare, insurance, financials and other businesses are infinitely more complex than say, serving up search results or 140 character micro blog posts.

It seems much easier in my opinion to scale a single function (search or tweet) than the kinds of tasks that a healthcare company has to do like say...scanning faxes from doctors, applying OCR and properly placing them into a pharmacy order system.