What have been Obama's biggest mistakes? Twitter users get an answer.

Marshall McLuhan, the 1960s media philosopher, is still right: The medium is the message.

President Obama held his first Twitter town hall Wednesday, and that’s the headline – not anything that was said. Mr. Obama started by, as he put it, making history as the first president to “live tweet.” And he kept his microblog post under the requisite 140 characters: “in order to reduce the deficit, what costs would you cut and what investments would you keep - bo,” Mr. Obama typed into the laptop on stage in the White House’s East Room.

He said he tweeted that question, because “it’s important to get the whole country involved” in the spirited debate going on in Washington over the national debt.

Over the hour-plus-long session, in which the president answered tweeted questions from around the country selected by Twitter, Obama pounded home his usual themes: The nation must invest in infrastructure, education, and clean energy even as it cuts spending elsewhere. Tax breaks for corporate jets and subsidies for the oil and gas industries are unacceptable. Modest tax increases on wealthy individuals won’t hurt job growth.

But his answers were not limited to 140 characters. He gave his usual discursive responses, which staff translated into Twitterese and posted.

Obama: my two biggest mistakes

In answer to a question, Obama admitted to two mistakes when he first came into office: not doing a better job of preparing the American people for the challenge of pulling out of the recession; and not anticipating how bad the housing crisis would get.

“Of all the things we've done,” he said, “that's probably been the area that's been most stubborn to us trying to solve the problem.”

Another tweeter asked Obama if he would issue an executive order to raise the debt ceiling, in line with the 14th Amendment to the Constitution. Obama said he hoped the impasse over the debt would not reach that point – but he did not rule it out.

"I don’t think we should even get to the constitutional issue," Obama said. "Congress has a responsibility to make sure we pay our bills. We've always paid them in the past. The notion that the US is going to default on its debt is just irresponsible, and my expectation is that over the next week to two weeks that Congress, working with the White House, comes up with a deal that solves our deficit, solves our debt problems, and makes sure that our full faith and credit is protected."

Since last Thursday, when the White House announced the Twitter town hall, more than 60,000 tweets came in using the hashtag #AskObama. Data from a Texas-based firm, Mass Relevance, helped make sure the questions were regionally and topically diverse, though the event was advertised as focused on jobs and the economy. In addition, a team of eight “seasoned Twitter users,” as the White House put it, from around the country flagged questions from their communities through retweets. Officials from Twitter selected the questions; Twitter co-founder Jack Dorsey moderated the event.

'Tweetup' roundup

The East Room audience consisted of 140 “characters” – pun intended – including 30 followers of the White House Twitter feed, who were invited to come to Washington and attend the event, dubbed a “Tweetup,” at their own expense.

Increasingly, the White House uses Twitter to release information to both the press and the public. Vice President Joe Biden (or rather, surrogates under the handle @VP) began tweeting on July 4. On Tuesday, White House communications officials told reporters they felt a presidential Twitter town hall made sense on the heels of Facebook and YouTube events.

“When we initially reached out to [Twitter], it was with a very open idea of how we could have an exciting event to bring people into the conversation of economy and jobs,” said Macon Phillips, White House director of new media. “They took that idea and ran with it.”

Clearly, demographics were a consideration as Obama heads into a tough reelection fight.

“It’s a pretty young room,” Obama said as he looked out over the audience.

A recent survey of White House Twitter followers showed that 38 percent are under the age of 35, according to an administration official.