The presidential candidates' discussion on domestic policy - moderated by PBS's Jim Lehrer from the University of Denver - has become the most tweeted event in US political history, according to the social network's communications team.
This year's Republican National Convention and Democratic National Convention were both previous holders of this record.
Statistics released on @gov - the account for Twitter's Government and Politics team - showed that there were over 2 million tweets about the presidential debate in the 24 hours before it got underway.
The total number of tweets for all four presidential and vice-presidential debates in 2008 was given as 500,000.
Romney's comments about cutting funding to PBS also engaged Twitter users, with a peak of 10,000 Tweets per minute mentioning the public service broadcaster.
Around 17,000 tweets per minute also mentioned Sesame Street character Big Bird, perhaps in response to Romney's declaration that he "loved" the character in spite of his desire to get rid of PBS's subsidy.
A number of Big Bird parody accounts - such as @firedbigbird and @bigbirdromney - were also created on Twitter within minutes of the Republican candidate's comments.