That figure, from the Pew Research Center's latest report on U.S. news habits, is a fraction of the number of Americans who are on Facebook. Nearly two-thirds of the country's adults are Facebook users — and that doesn't count the young teens that Facebook depends on as a core demographic.
Twitter's low penetration rate makes its rapid insinuation into media and culture all the more impressive. A whole cottage industry has sprung up around writing articles about tweets and the people who tweet them. With the company set to IPO at a valuation of $13.6 billion, Twitter is soon going to become much more of a household name.
But you could also say that given Twitter's remarkably strong brand, getting more people to adopt will be one of the company's key challenges moving forward. Dangling access to news in front of them probably isn't going to work; most people who are information omnivores are already there. In that light, Twitter's move toward a bigger role during television events seems increasingly smart.