I also believe that the cloud has proven people are willing to forgo some privacy in exchange for the ease of use/access. I don't take part is such activities, but many seem to.
Or they don't understand exactly what they are trading away in exchange for the free service.
Which would be understandable, since it's not like any of those services go out of their way to explain it to them. It's just marketed as a free lunch.
Of course, it does mention documents and such as well, but I'm sure there are plenty of uses for "socially" sharing that type of thing too.