In 2008, Mark Zuckerberg told a tech-conference audience that the amount of information people shared seemed to be doubling every year.

It was a bold prediction. It just doesn't seem to be coming true.

If anything, people are undersharing—and that poses a massive challenge to the Facebook CEO's vision of Graph Search.

Zuckerberg unveiled Graph Search, a considerable upgrade to Facebook's notoriously weak search feature, last week, to much fanfare.