The simple fact that you're reading this story puts you in the other category;  One-fifth of adults living in the United States don't use the internet.

Internet adoption among U.S. adults increased rapidly from the mid-’90s to about 2005. Since then, though, the number of adult Internet users has remained almost stable at around 75 to 80%. The Pew Internet & American Life Project’s latest poll shows that this trend continued in 2011. Those who are online use the Internet more than ever before, but about one in five U.S. adults is simply not online.

Part of the disparity falls along the lines of household income.

Virtually every U.S. household with an annual income over $75,000 is online, but that’s only true for 63% of adults who live in a household with an annual income under $30,000. The numbers look quite similar for different education levels: 94% of adults with post-graduate degrees are online, but 57% of those without high school diplomas remain offline.

However, it's not just income that keeps some adults from using the internet.  Some people just choose not to.

Beside the obvious economic barriers to entry, though, the Pew poll also found that half of those who don’t go online do so because they just don’t think “the Internet is relevant to them.” One in five of those who are not online today think that they just don’t know enough about technology to use the Internet on their own.

Report: One In Five U.S. Adults Does Not Use The Internet | TechCrunch.