The vast majority of Americans who are following the Olympics are doing so on TV

"Large majorities of Americans are following coverage of the Olympic Games in London. Nearly eight-in-ten (78%) say they have watched or followed Olympic coverage either on television, online or on social networks.
Television remains far-and-away the leading platform for Olympic coverage; 73% say they have watched coverage on television. Still, 17% say they have watched online or digitally and 12% report they have followed Olympic coverage on social networking sites like Facebook or Twitter. Most Olympic followers (68%) say they are watching events in the evening after they have already occurred. At the same time, almost a quarter (23%) say they are watching live during the day.
The latest national survey by the Pew Research Center for the People & the Press and the Pew Internet & American Life Project, conducted August 2-5, 2012 among 1,005 adults, finds that NBC’s coverage of the Olympics receives high marks from the public."
Source:  Research by Pew Internet & American Life Project, 6th August 2012