Zuckerberg shared this incredible metric with Tim O'Reilly and John Battelle in a panel discussion at the Web 2.0 Summit. The Facebook CEO believes most industries are going to be redesigned around users' social activity; he used this metric as a foundation for that statement.
Zuckerberg also revealed the company's recently launched Groups product "has been one of the fastest growing products ever."
His clear point is that Facebook builds products -- Photos, Groups, Messages -- that people use more frequently than they use more established services with similar features because Facebook's products are inherently social. Zuckerberg also stated he believes the company will play a significant role in reforming games, music, music, TV and news verticals going forward.
For companies that are not social, Zuckerberg emphatically told them, "Get on the bus."