According to the most recent numbers from Nielsen, visitors to Facebook’s website spent 6:40:32 (hh:mm:ss) on the site in September. Back in March, visitors spent 7:09:23 on the social network. This probably isn’t a very troubling sign though, since the decline is probably attributable to the emergence of mobile usage. According to Facebook, it has 604 million monthly active mobile users, about 60 percent of its overall user base. Nielsen also found that the average U.S. Internet user spent 28:32:52 online in September, and that the total number of people who want online was 212.2 million. (Marketing Land)