Email, Facebook, cat videos — these are just a few of the things that 15 percent of American adults are missing out on every day because they don't use the Internet.

However, that 15 percent is a huge reduction from the percentage of Americans who did not use the Internet in 2000, according to a new analysis of survey data by the Pew Research Center, a nonprofit organization in Washington, D.C. In that year, almost half of all Americans (48 percent) said they didn't go online.

The number of people in the United States who use the Internet increased steadily from 2000 until 2012, when the percentage of offline adults fell to 15 percent. Since then, despite efforts by the government and social service organizations to encourage Americans to get online, that number hasn't budged, according to Pew. [10 Technologies That Will Transform Your Life]

Why are some Americans so reluctant to sign on? A third of those surveyed who aren't online (34 percent) said they don't think the Internet is relevant to their lives, or that they're simply not interested in what the Web has to offer. Another 32 percent of people who don't use the Internet said the technology required to access the Internet is just too tough to get the hang of, and 8 percent said they were "too old to learn."

But some people said they don't use the Internet because they cannot afford to do so, according to Pew. The survey data showed that 19 percent of those not online cited the expense of Internet service or owning a computer as their reason for staying offline.

Certain groups are more likely than others to go without Internet, the Pew researchers found. Seniors are the most likely to say they don't go online — 39 percent of people age 65 and older don't use the Internet. In comparison, just 3 percent of 18- to 29-year-olds don't use the Internet.

One-third of American adults who have less than a high school education are also offline, and as a person's education level rises, so does the likelihood that he or she uses the Internet, according to Pew.

Where people live and how much they earn are also indicators of how likely they are to stay offline, the researchers found. Those living in rural areas are nearly twice as likely as city dwellers and suburbanites to say that they never use the Internet. And Americans in households earning less than $30,000 a year are about eight times more likely than the most affluent adults to say they don't go online.

Differences in Internet usage are also apparent across racial and ethnic lines. Fourteen percent of white people surveyed by Pew said they don't use the Internet, but 18 percent of Hispanics and 20 percent of black people in the U.S. don't use the Internet, the researchers found. The group least likely to go without Internet is English-speaking Asian Americans — only 5 percent of people in this group said they don't go online.

However, the researchers also said older adults are more likely to use the Internet than in 2000: In that year, 86 percent of those 65 and older said they didn't use the Internet, but today, that figure has been cut in half. Now that the Internet has been around for several decades, it's possible that many of today's seniors learned how to use this technology in their younger years and are simply continuing to use the Internet as they age.

And although 33 percent of Americans with less than a high school education still don't go online in 2015, there were far fewer Internet users among this group in 2000, when 81 percent of Americans without a high school diploma said they never went online.

Follow Elizabeth Palermo @techEpalermo. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.