I think most of it is about the brand; about buying a name which is already imprinted in millions of minds. The technology alone is probably only worth 1-5% (?) of the price.
2. Create Stack overflow and make it the goto for developers.
3. Create todo list
etc..
The brand was there before they even started the todo list. All it took was a blog post "We are going to create a todo list now" and it was pretty much set.
