During its video calling announcement today (you can watch the whole thing here), Facebook co-founder and CEO Mark Zuckerberg announced that users are sharing 4 billion things on the social network every day. He also noted that the site's users are sharing content at an exponential rate.
On average, every Facebook user today shares twice the amount of items they shared a year ago. Based on the data Facebook sees, one year from now the average Facebook user will probably be sharing twice as much as her or she is today. Zuckerberg did note that this trend, while important, likely applies to the Internet in general, rather than just Facebook.
Zuckerberg claims that the law of sharing is thus y = 2^x. Furthermore, he says that "we are at the elbow of the curve." In other words, sharing could be about to explode.
As a result, Zuckerberg noted that future Facebook apps will be dramatically different and will make sharing growth exponential in the coming years. He didn't elaborate on what those new apps would do to increase sharing, but hinted that it would be a different social experience that included passive apps that share things on a users' behalf (think music, TV shows, movies, and so on).
The reason Zuckerberg brought this up is because he believes the amount of sharing per person rather than the number of users is the metric to watch now. This is why Facebook only today noted that it had 750 million users, rather than announcing it separately the day the milestone was reached.