According to Nielsen 57 million people watched the presidential debate on Friday night between Barack Obama and John McCain, meaning about 1/3 of the television sets in major U.S. markets tuned in — also based on our unofficial research (i.e. Twitter) a whole lot of television sets in bars. That is just shy of the number of people who watched the first debate between John Kerry and George W. Bush in 2004 and well under the most-watched presidential debate back in 1980 — also known as the pre-HBO, Internet era — between Ronald Reagan and Jimmy Carter, which drew more than 80 million viewers.

Of course 57 million doesn’t take into account the number of people who were watching the debate via a livestream on the Internet, or the ever-proliferating liveblog, or the jam-packed twitter highway. At some point someone is going to have to figure out how to measure these things and incorporate them — does the fact the Palin-Couric SNL opener vid on NBC.com has already had 1,202,068 view factor into the ratings at all? Here’s what we do know.

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