According to the U.S. Census Bureau’s official “Population Clock,” around 311 million people live in the U.S. at this moment.  Now here’s the interesting thing: new data from Edison Research and Arbitron indicates that more than half of Americans over the age of 11 have a Facebook account.

Edison and Arbitron conducted a national study of 2,020 individuals in order to arrive that conclusion, so don’t think the finding was based on a survey of 20 folks in Silicon Valley.  Also, this isn’t the first time the two groups have posed questions on the subject.

Tom Webster, Vice President of Strategy and Marketing at Webster, explained in a short blog post, “We have been tracking the growth of Facebook since 2008, and have watched it go from 8% usage just three years ago, to 51% today.”

That’s kind of stunning.  Indeed, it doesn’t take a math expert to figure out that absolutely everyone in America would soon have a Facebook account if the social network was somehow able to maintain that growth rate for a little while longer.

That should help explain why Facebook is focusing so much on overseas growth and interesting new initiatives like Facebook Places; there’s perhaps not a lot left to achieve when it comes to just getting Americans to sign up.

Anyway, we wouldn’t be surprised if this news causes a few more advertisers to try out Facebook, and investors may scramble (yet again) to buy shares of the company on the secondary market.

It seems another announcement about Facebook’s official user count shouldn’t be too far away, either.