How Americans spend their time online has shifted dramatically over the past 12 months, according to a new report.

Social networks dominated by Facebook now account for 22.7 percent of the hours spent in front of a computer, a leap of 43 percent over last year's figures.

According to the Neilson Co. report, the shift to social networks for communication caused a precipitous drop in email and instant messaging. Time spent emailing was down 28 percent and instant messaging dropped 15 percent. 

Given a typical hour, Nielsen says the average user will spend 13 minutes on Facebook and other social networks, six minutes playing games, five minutes with email, 2 minutes each on browsing Internet portals like Yahoo, instant messaging, watching videos, conducting searches, and reading software information for a total of 36 minutes. The balance of the hour is spent on 77 additional categories measured by the firm, all of which take 2 minutes or less.

Facebook offers direct messages as well as instant chats, so it may be that the 500 million Facebook users are opting to use a single service rather than additional email and IM services.

The Nielsen study notes their findings do not signal the death of email. Rather, email has moved to mobile. Email captures 25 minutes of every hour on a smartphone, more than four times the 6 minutes spent on social networks.