As you may recall, on August 1, 2014, Facebook went down for about one hour. The analytics company Chartbeat used the outage as an opportunity to study what the web would look like without Facebook. Overall, there was a three percent drop in usage, but mobile use saw an 8.5 percent drop. In other words, lots of mobile web traffic moves through Facebook, i.e., users go through Facebook rather than other homepages. As web traffic increasingly goes mobile, this study shows the importance of Facebook.
You can read more about it at Buzzfeed here.