This number grew from 58 million earlier in the day to 67 million to 70 million. As this piece points out, this number could be quite low because of the various digital platforms where people can watch, and out of home TV viewing not measured. Interestingly, 70 million Americans watched the first Kennedy/Nixon debate in 1960, when there were 180 million Americans, compared with more than 300 million today. So, a massive percentage of voting age Americans watched that debate.
Of course, there was nothing else to watch back then. Your only viewing options were the three networks and they all carried it. Viewers had virtually countless options other than the debate last night. Obama/Romney is the highest since 1980. And again, there were no digital platforms on which to watch it back then and TV was still largely dominated by the Big Three in '80.